Stable and efficient electroluminescent materials

ABSTRACT

An organic light emitting device is provided. The device has an anode, a cathode, and an emissive layer disposed between the anode and the cathode. The emissive layer may include a molecule of Formula I wherein an alkyl substituent at position R′ 5  results in high efficiency and operational stability in the organic light emitting device. Additionally or alternatively, the emissive layer may include a metal-ligand complex in which the ligand is an aryl or alkyl substituted phenylpyridine ligand.

REFERENCE TO RELATED APPLICATION

This application is related to and claims priority from ProvisionalApplication Ser. No. 60/682,690 filed May 19, 2005.

RESEARCH AGREEMENT

The claimed invention was made by, on behalf of, and/or in connectionwith one or more of the following parties to a joint universitycorporation research agreement: Princeton University, The University ofSouthern California, and the Universal Display Corporation. Theagreement was in effect on and before the date the claimed invention wasmade, and the claimed invention was made as a result of activitiesundertaken within the scope of the agreement.

FIELD OF THE INVENTION

The present invention relates to organic light emitting devices (OLEDs),and more specifically to phosphorescent organic materials used in suchdevices. More specifically, the present invention relates tophosphorescent emitting materials with improved electroluminescentefficiency when incorporated into an OLED.

BACKGROUND

Opto-electronic devices that make use of organic materials are becomingincreasingly desirable for a number of reasons. Many of the materialsused to make such devices are relatively inexpensive, so organicopto-electronic devices have the potential for cost advantages overinorganic devices. In addition, the inherent properties of organicmaterials, such as their flexibility, may make them well suited forparticular applications such as fabrication on a flexible substrate.Examples of organic opto-electronic devices include organic lightemitting devices (OLEDs), organic phototransistors, organic photovoltaiccells, and organic photodetectors. For OLEDs, the organic materials mayhave performance advantages over conventional materials. For example,the wavelength at which an organic emissive layer emits light maygenerally be readily tuned with appropriate dopants.

As used herein, the term “organic” includes polymeric materials as wellas small molecule organic materials that may be used to fabricateorganic opto-electronic devices. “Small molecule” refers to any organicmaterial that is not a polymer, and “small molecules” may actually bequite large. Small molecules may include repeat units in somecircumstances. For example, using a long chain alkyl group as asubstituent does not remove a molecule from the “small molecule” class.Small molecules may also be incorporated into polymers, for example as apendent group on a polymer backbone or as a part of the backbone. Smallmolecules may also serve as the core moiety of a dendrimer, whichconsists of a series of chemical shells built on the core moiety. Thecore moiety of a dendrimer may be a fluorescent or phosphorescent smallmolecule emitter. A dendrimer may be a “small molecule,” and it isbelieved that all dendrimers currently used in the field of OLEDs aresmall molecules. In general, a small molecule has a well-definedchemical formula with a single molecular weight, whereas a polymer has achemical formula and a molecular weight that may vary from molecule tomolecule.

OLEDs make use of thin organic films that emit light when voltage isapplied across the device. OLEDs are becoming an increasinglyinteresting technology for use in applications such as flat paneldisplays, illumination, and backlighting. Several OLED materials andconfigurations are described in U.S. Pat. Nos. 5,844,363, 6,303,238, and5,707,745, which are incorporated herein by reference in their entirety.

OLED devices are generally (but not always) intended to emit lightthrough at least one of the electrodes, and one or more transparentelectrodes may be useful in an organic opto-electronic device. Forexample, a transparent electrode material, such as indium tin oxide(ITO), may be used as the bottom electrode. A transparent top electrode,such as disclosed in U.S. Pat. Nos. 5,703,436 and 5,707,745, which areincorporated by reference in their entireties, may also be used. For adevice intended to emit light only through the bottom electrode, the topelectrode does not need to be transparent, and may be comprised of athick and reflective metal layer having a high electrical conductivity.Similarly, for a device intended to emit light only through the topelectrode, the bottom electrode may be opaque and/or reflective. Wherean electrode does not need to be transparent, using a thicker layer mayprovide better conductivity, and using a reflective electrode mayincrease the amount of light emitted through the other electrode, byreflecting light back towards the transparent electrode. Fullytransparent devices may also be fabricated, where both electrodes aretransparent. Side emitting OLEDs may also be fabricated, and one or bothelectrodes may be opaque or reflective in such devices.

As used herein, “top” means furthest away from the substrate, while“bottom” means closest to the substrate. For example, for a devicehaving two electrodes, the bottom electrode is the electrode closest tothe substrate, and is generally the first electrode fabricated. Thebottom electrode has two surfaces, a bottom surface closest to thesubstrate, and a top surface further away from the substrate. Where afirst layer is described as “disposed over” a second layer, the firstlayer is disposed further away from substrate. There may be other layersbetween the first and second layer, unless it is specified that thefirst layer is “in physical contact with” the second layer. For example,a cathode may be described as “disposed over” an anode, even thoughthere are various organic layers in between.

As used herein, “solution processible” means capable of being dissolved,dispersed, or transported in and/or deposited from a liquid medium,either in solution or suspension form.

As use herein, a first “Highest Occupied Molecular Orbital” (HOMO) or“Lowest Unoccupied Molecular Orbital” (LUMO) energy level is “greaterthan” a second HOMO or LUMO energy level if the first energy level iscloser to the vacuum energy level. Since ionization potentials (IP) aremeasured as a negative energy relative to a vacuum level, a greater HOMOcorresponds to an IP having a smaller absolute value (an IP that is lessnegative). Similarly, a greater LUMO corresponds to an electron affinity(EA) having a smaller absolute value (an EA that is less negative). On aconventional energy level diagram, with the vacuum level at the top, theLUMO of a material is higher than the HOMO of the same material. A“greater” HOMO or LUMO appears closer to the top of such a diagram thana “lesser” HOMO or LUMO.

One application for phosphorescent emissive molecules is a full colordisplay. Industry standards for such a display call for pixels adaptedto emit particular colors, referred to as “saturated” colors. Inparticular, these standards call for saturated red, green, and bluepixels. Color may be measured using CIE coordinates, which are wellknown to the art.

One example of a green emissive molecule is tris(2-phenylpyridine)iridium, denoted Ir(ppy)₃, which has the structure:

In this, and later figures herein, the dative bond from nitrogen tometal (here, Ir) is depicted in metal complexes as a straight line.Ir(ppy)₃ emits a spectrum at CIE 0.30, 0.63, and has a half-life ofabout 10,000 hours at an initial luminance of 500 cd/m², and a quantumefficiency of about 6%. Kwong et al., Appl. Phys. Lett., 81, 162 (2002).

Industry standards call for the lifetime of full color displays to be atleast about 5000 hours. In addition, high stability and efficiency areimportant characteristics of high quality displays. These requirementshave helped generate a need for phosphorescent emissive materials thatexhibit longer lifetimes, higher stability, and higher efficiency in thered, green and blue wavelength regimes than have been achieved in theprior art. Phosphorescent materials with improved device efficiency andstability are disclosed herein.

SUMMARY OF THE INVENTION

An organic light emitting device is provided. The device has an anode, acathode, and an emissive layer disposed between the anode and thecathode. The emissive layer further comprises an emissive materialhaving the structure:

Wherein

-   -   M is a metal selected from Ir, Pt, Rh or Pd;    -   m is a value from 1 to the maximum number of ligands that may be        attached to the metal;    -   m+n is the maximum number of ligands that may be attached to the        metal;    -   (X—Y) is an ancillary ligand;    -   ring A is an aromatic heterocyclic or a fused aromatic        heterocyclic ring having an alkyl substituent at the R′₅        position and having at least one nitrogen atom, N, that is        coordinated to the metal M, wherein the ring A can be optionally        substituted with one or more substituents at the R′₃, R′₄ and        R′₆ positions;        -   additionally or alternatively the R′₃ and R′₄ substituted            positions on ring A together form, independently a fused            ring, wherein the fused ring may be optionally substituted;    -   ring B is an aromatic ring with at least one carbon atom        coordinated to metal M, wherein ring B can be optionally        substituted with one or more substituents at the R₃, R₄, R₅, and        R₆ positions;    -   R′₃ R′₄ and R′₆ are each independently H, alkyl, alkenyl,        alkynyl, heteroalkyl, alkenyl, alkynyl, heteroalkyl, aryl,        heteroaryl, aralkyl; and wherein R′₃ R′₄ and R′₆ are optionally        substituted by one or more substituents Z; and    -   R₃, R₄, R₅ and R₆ are each independently selected from the group        consisting of H, alkyl, alkenyl, alkynyl, alkylaryl, CN, CO₂R,        C(O)R, NR₂, NO₂, OR, halo, aryl, heteroaryl, substituted aryl,        substituted heteroaryl or a heterocyclic group        -   such that when R′₃, R′₄, and R′₆ are all H, R₃, R₄, R₅, and            R₆ are also all H or at least one of R₄, R₅ and R₆ is a            linking group covalently linking two or more of the maximum            number of ligands that may be attached to the metal, an            unsubstituted phenyl ring, a fluoro-substituted phenyl ring            or a phenyl ring substituted with a substituent that renders            the phenyl ring equally or less coplanar than the            unsubstituted phenyl ring with respect to Ring B;    -   alternatively, R′₃ and R₆ may be bridged by a group selected        from —CR₂—CR₂—, —CR═CR—, —CR₂—, —O—, —NR—, —O—CR₂—, —NR—CR₂—,        and —N═CR—;    -   each R is independently H, alkyl, alkenyl, alkynyl, heteroalkyl,        aryl, heteroaryl, or aralkyl; wherein R is optionally        substituted by one or more substituents Z;    -   each Z is independently a halogen, R′, OR′, N(R′)₂, SR′, C(O)R′,        C(O)OR′, C(O)N(R′)₂, CN, SO₂, SOR′, SO₂R′, or SO₃R′; and    -   each R′ is independently H, alkyl, alkenyl, alkynyl,        heteroalkyl, aryl, or heteroaryl;

The emissive material itself is also provided. The emissive material mayhave improved efficiency and stability when incorporated into a lightemitting device. In particular, the devices of the present invention mayexhibit dramatically improved efficiency over known devices.

In another embodiment the emissive layer comprises an emissive materialhaving the structure:

wherein

-   -   M is a metal selected from Ir, Pt, Rh or Pd;    -   (X—Y) is an ancillary ligand;    -   m is a value from 1 to the maximum number of ligands that may be        attached to the metal;    -   and m+n is the maximum number of ligands that may be attached to        the metal;    -   ring A is an aromatic heterocyclic or a fused aromatic        heterocyclic ring having at least one nitrogen atom, N, that is        coordinated to the metal M, wherein the ring A can be optionally        substituted with one or more substituents at the R′₃, R′₄ R′₅        and R′₆ positions;        -   additionally or alternatively the R′₃ and R′₄ substituted            positions on ring A together form, independently a fused            ring, wherein the fused ring may be optionally substituted;    -   ring B is an aromatic ring with at least one carbon atom        coordinated to metal M, wherein ring B can be optionally        substituted with one or more substituents at the R₃, R₄, R₅, and        R₆ positions;    -   R′₃ R′₄ R′₅ and R′₆ are each independently H, alkyl, alkenyl,        alkynyl, heteroalkyl, alkenyl, alkynyl, heteroalkyl, aryl,        heteroaryl, aralkyl; and wherein R′₃ R′₄ and R′₆ are optionally        substituted by one or more substituents Z; and    -   R₃, R₄, R₅ and R₆ are each independently selected from the group        consisting of H, alkyl, alkenyl, alkynyl, alkylaryl, CN, CO₂R,        C(O)R, NR₂, NO₂, OR, halo, aryl, heteroaryl, substituted aryl,        substituted heteroaryl or a heterocyclic group    -   alternatively, R′₃ and R₆ may be bridged by a group selected        from —CR₂—CR₂—, —CR═CR—, —CR₂—, —O—, —NR—, —O—CR₂—, —NR—CR₂—,        and —N═CR—;    -   each R is independently H, alkyl, alkenyl, alkynyl, heteroalkyl,        aryl, heteroaryl, or aralkyl; wherein R is optionally        substituted by one or more substituents Z;    -   each Z is independently a halogen, R′, OR′, N(R′)₂, SR′, C(O)R′,        C(O)OR′, C(O)N(R′)₂, CN, SO₂, SOR′, SO₂R′, or SO₃R′; and    -   each R′ is independently H, alkyl, alkenyl, alkynyl,        heteroalkyl, aryl, or heteroaryl;    -   wherein at least one of R₃, R₄, R₅, R₆ R′₃ R′₄ R′₅ and R′₆ is an        alkyl or aryl substituent.

The emissive material in this embodiment is also provided. The emissivematerial may have improved efficiency and stability when incorporatedinto a light emitting device.

In preferred embodiments, the emissive material is a tris compound(where m=3).

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an organic light emitting device having separate electrontransport, hole transport, and emissive layers, as well as other layers.

FIG. 2 shows an inverted organic light emitting device that does nothave a separate electron transport layer.

FIG. 3 shows the plots comparing current density (mA/cm²) vs. voltage(V) in devices with either 100 Å ofaluminum(III)bis(2-methyl-8-hydroxyquinolinato)-4-phenylphenolate (BAlq)as the ETL2 (Experimental Device 1) or 100 Å of HPT as the ETL2(Experimental Device 2) using Ir(5′-Meppy)₃ as the emissive materialdoped at 6% (all dopant concentrations are in wt % unless otherwisespecified).

FIG. 4 shows the plots comparing luminous efficiency (cd/A) vs.brightness (cd/m²) comparing devices with either 100 Å ofaluminum(III)bis(2-methyl-8-hydroxyquinolinato)4-phenylphenolate (BAlq)as the ETL2 (Experimental Device 1) or 100 Å of HPT as the ETL2(Experimental Device 2) using Ir(5′-Meppy)₃ as the emissive materialdoped at 6%.

FIG. 5 shows the external quantum efficiency (η_(ext)) as a function ofcurrent density (mA/cm2) comparing devices with either 100 Å ofaluminum(III)bis(2-methyl-8-hydroxyquinolinato)4-phenylphenolate (BAlq)as the ETL2 (Experimental Device 1) or 100 Å of HPT as the ETL2(Experimental Device 2) using Ir(5′-Meppy)₃ as the emissive materialdoped at 6%.

FIG. 6 shows the normalized electroluminescence spectra (normalized ELintensity vs. wavelength) at a current density of 10 mA/cm² comparingdevices with either 100 Å ofaluminum(III)bis(2-methyl-8-hydroxyquinolinato)4-phenylphenolate (BAlq)as the ETL2 (Experimental Device 1) or 100 Å of HPT as the ETL2(Experimental Device 2) using Ir(5′-Meppy)₃ as the emissive materialdoped at 6%.

FIG. 7 shows the normalized luminance decay comparing devices witheither 100 Å ofaluminum(III)bis(2-methyl-8-hydroxyquinolinato)4-phenylphenolate (BAlq)as the ETL2 (Experimental Device 1) or 100 Å of HPT as the ETL2(Experimental Device 2) using Ir(5′-Meppy)₃ as the emissive materialdoped at 6% under constant current drive of 40 mA/cm² at roomtemperature.

FIG. 8 shows the plots of the current density (mA/cm²) vs. the voltage(V) comparing devices using 100 Å ofaluminum(III)bis(2-methyl-8-hydroxyquinolinato)4-phenylphenolate (BAlq)as the ETL2 and using Ir(5′-Me-5-Phppy)₃ as the emissive material dopedat 6% (Experimental Device 3), 8% (Experimental Device 5), 10%(Experimental Device 7) and 12% (Experimental Device 9).

FIG. 9 shows the plots of luminous efficiency (cd/A) vs. brightness(cd/m²) for devices using 100 Å ofaluminum(III)bis(2-methyl-8-hydroxyquinolinato)4-phenylphenolate (BAlq)as the ETL2 and using I Ir(5′-Me-5-Phppy)₃ as the emissive materialdoped at 6% (Experimental Device 3), 8% (Experimental Device 5), 10%(Experimental Device 7) and 12% (Experimental Device 9).

FIG. 10 shows the external quantum efficiency (η_(ext)) as a function ofcurrent density (mA/cm²) for devices using 100 Å ofaluminum(III)bis(2-methyl-8-hydroxyquinolinato)4-phenylphenolate (BAlq)as the ETL2 and using Ir(5′-Me-5-Phppy)₃ as the emissive material dopedat 6% (Experimental Device 3), 8% (Experimental Device 5), 10%(Experimental Device 7) and 12% (Experimental Device 9).

FIG. 11 shows the normalized electroluminescence spectra (normalized ELintensity vs. wavelength) at a current density of 10 mA/cm² for devicesusing 100 Å ofaluminum(III)bis(2-methyl-8-hydroxyquinolinato)4-phenylphenolate (BAlq)as the ETL2 and using Ir(5′-Me-5-Phppy)₃ as the emissive material dopedat 6% (Experimental Device 3), 8% (Experimental Device 5), 10%(Experimental Device 7) and 12% (Experimental Device 9).

FIG. 12 shows the plots of the current density (mA/cm²) vs. the voltage(V) comparing devices using 50 Å of HPT as the ETL2 and usingIr(5′-Me-5-Phppy)₃ as the emissive material doped at 6% (ExperimentalDevice 4), 8% (Experimental Device 6), 10% (Experimental Device 8) and12% (Experimental Device 10).

FIG. 13 shows the plots of luminous efficiency (cd/A) vs. brightness(cd/m²) comparing devices using 50 Å of HPT as the ETL2 and usingIr(5′-Me-5-Phppy)₃ as the emissive material doped at 6% (ExperimentalDevice 4), 8% (Experimental Device 6), 10% (Experimental Device 8) and12% (Experimental Device 10).

FIG. 14 shows the external quantum efficiency (η_(ext)) as a function ofcurrent density (mA/cm²) comparing devices using 50 Å of HPT as the ETL2and using Ir(5′-Me-5-Phppy)₃ as the emissive material doped at 6%(Experimental Device 4), 8% (Experimental Device 6), 10% (ExperimentalDevice 8) and 12% (Experimental Device 10).

FIG. 15 shows the normalized electroluminescence spectra (normalized ELintensity vs. wavelength) at a current density of 10 mA/cm² comparingdevices using 50 Å of HPT as the ETL2 and using Ir(5′-Me-5-Phppy)₃ asthe emissive material doped at 6% (Experimental Device 4), 8%(Experimental Device 6), 10% (Experimental Device 8) and 12%(Experimental Device 10).

FIG. 16 shows the normalized luminance decay comparing devices witheither 100 Å ofaluminum(III)bis(2-methyl-8-hydroxyquinolinato)4-phenylphenolate (BAlq)as the ETL2 (Experimental Device 5) or 50 Å of HPT as the ETL2(Experimental Device 6) using Ir(5′-Me-5-Phppy)₃ as the emissivematerial doped at 8% under constant current drive of 40 mA/cm² at roomtemperature.

FIG. 17 shows the normalized electroluminescence spectra (normalized ELintensity vs. wavelength) at a current density of 10 mA/cm² forExperimental devices 11-16 usingfac-tris[3-methyl-5,6-dihydrobenzo[h]quinolinato-N,C2′]iridium(III)(Compound Example V) as the emissive material doped at 6%-10% in CBP.

FIG. 18 shows the plots of luminous efficiency (cd/A) vs. brightness(cd/m²) comparing devices with either 100 Å ofaluminum(III)bis(2-methyl-8-hydroxyquinolinato)4-phenylphenolate (BAlq)as the ETL2 (Experimental Devices 11, 13 and 15) or 50 Å of HPT as theETL2 (Experimental Devices 12, 14 and 16) usingfac-tris[3-methyl-5,6-dihydrobenzo[h]quinolinato-N,C2′]iridium(III)(Compound Example V) as the emissive material doped at 6%-10% in CBP.

FIG. 19 shows the plots of the current density (mA/cm²) vs. the voltage(V) comparing devices with either 100 Å ofaluminum(III)bis(2-methyl-8-hydroxyquinolinato)4-phenylphenolate (BAlq)as the ETL2 (Experimental Devices 11, 13 and 15) or 50 Å of HPT as theETL2 (Experimental Devices 12, 14, and 16) usingfac-tris[3-methyl-5,6-dihydrobenzo[h]quinolinato-N,C2′]iridium(I)(Compound Example V) as the emissive material doped at 6%-10% in CBP.

FIG. 20 shows the normalized luminance decay for Experimental Devices 13and 14.

FIG. 21 shows the normalized electroluminescence spectra (normalized ELintensity vs. wavelength) at a current density of 10 mA/cm² forExperimental Devices 17-22 usingfac-tris[2-(2′-methylbiphenyl-3-yl)pyridinato-N,C2]iridium(III)(Compound Example VI) in the emissive layer doped at 6%-12% in CBP.

FIG. 22 shows the plots of luminous efficiency (cd/A) vs. brightness(cd/m²) for devices Experimental Devices 18-22 comparing devices witheither 100 Å ofaluminum(III)bis(2-methyl-8-hydroxyquinolinato)4-phenylphenolate (BAlq)as the ETL2 (Experimental Devices 17, 19 and 21) or 50 Å of HPT as theETL2 (Experimental Devices 18, 20, and 22) usingfac-tris[2-(2′-methylbiphenyl-3-yl)pyridinato-N,C^(2′)]iridium(III)(Compound Example VI) as the emissive material doped at 6%-12% in CBP.

FIG. 23 shows the plots of the current density (mA/cm²) vs. the voltage(V) comparing devices with either 100 Å ofaluminum(III)bis(2-methyl-8-hydroxyquinolinato)4-phenylphenolate (BAlq)as the ETL2 (Experimental Devices 17, 19 and 21) or 50 Å of HPT as theETL2 (Experimental Devices 18, 20, and 22) usingfac-tris[2-(2′-methylbiphenyl-3-yl)pyridinato-N,C^(2′)]iridium(III)(Compound Example VI) as the emissive material doped at 6%-12% in CBP.

FIG. 24 shows the normalized luminance as a function of time at acurrent density of 40 mA/cm² for annealed Experimental Devices 19, 21and 22.

FIG. 25 shows the normalized luminance as a function of time at aninitial luminance of 1000 cd/m² for Experimental Devices 17, 19, 20 and22.

FIG. 26 shows the normalized electroluminescence spectra (normalized ELintensity vs. wavelength) at a current density of 10 mA/cm² forExperimental Devices 23-26 using a Hexadentate Ligand Complex (CompoundExample VII) as the emissive material in the emissive layer doped at6%-10% in CBP.

FIG. 27 shows the plots of luminous efficiency (cd/A) vs. brightness(cd/m²) comparing devices with either 100 Å ofaluminum(III)bis(2-methyl-8-hydroxyquinolinato)4-phenylphenolate (BAlq)as the ETL2 (Experimental Devices 23 and 25) or 50 Å of HPT as the ETL2(Experimental Devices 24 and 26) using a Hexadentate Ligand Complex(Compound Example VII) as the emissive material in the emissive layerdoped at 6%-10% in CBP.

FIG. 28 shows the plots of the current density (mA/cm²) vs. the voltage(V) comparing devices with either 100 Å ofaluminum(III)bis(2-methyl-8-hydroxyquinolinato)4-phenylphenolate (BAlq)as the ETL2 (Experimental Devices 23 and 25) or 50 Å of HPT as the ETL2(Experimental Devices 24 and 26) using a Hexadentate Ligand Complex(Compound Example VII) as the emissive material in the emissive layerdoped at 6%-10% in CBP.

FIG. 29 shows the normalized luminance as a function of time at acurrent density of 40 mA/cm² for Experimental Devices 23 and 26.

FIG. 30 shows the normalized electroluminescence spectra (normalized ELintensity vs. wavelength) at a current density of 10 mA/cm² inExperimental Devices 27 and 28 having a neat layer offac-tris[2-(2′-methylbiphenyl-3-yl)pyridinato-N,C^(2′)]iridium(III)(Compound Example VI) as the emissive layer and Comparative ExampleDevices 3 and 4 having a neat layer of Ir(3′-Meppy)₃ as the emissivelayer comparing devices with either 100 Å ofaluminum(III)bis(2-methyl-8-hydroxyquinolinato)4-phenylphenolate (BAlq)as the ETL2 (Experimental Device 27 and Comparative Example Device 3) or50 Å of HPT as the ETL2 (Experimental Device 28 and Comparative ExampleDevice 4)

FIG. 31 shows the plots of luminous efficiency (cd/A) vs. brightness(cd/m²) voltage comparing devices having neat emissive layers witheither 100 Å ofaluminum(III)bis(2-methyl-8-hydroxyquinolinato)4-phenylphenolate (BAlq)as the ETL2 (Experimental Device 27 and Comparative Example Device 3) or50 Å of HPT as the ETL2 (Experimental Device 28 and Comparative ExampleDevice 4).

FIG. 32 shows the plots of the current density (mA/cm²) vs. the voltage(V) having neat emissive layers with either 100 Å ofaluminum(III)bis(2-methyl-8-hydroxyquinolinato)4-phenylphenolate (BAlq)as the ETL2 (Experimental Device 27 and Comparative Example Device 3) or50 Å of HPT as the ETL2 (Experimental Device 28 and Comparative ExampleDevice 4).

FIG. 33 shows the plots of the current density (mA/cm²) vs. the voltage(V) comparing devices with either 100 Å ofaluminum(III)bis(2-methyl-8-hydroxyquinolinato)4-phenylphenolate (BAlq)as the ETL2 (Experimental Devices 29, 31 and 33) or 50 Å of HPT as theETL2 (Experimental Devices 30, 32 and 34) using Ir[5′-Me-5-(4-FPh)ppy]₃(Compound Example VIII) as the emissive material in the emissive layerdoped at 6%, 8% and 10% in CBP.

FIG. 34 shows the plots of the luminous efficiency (cd/A) vs. brightness(cd/m²) comparing devices with either 100 Å ofaluminum(III)bis(2-methyl-8-hydroxyquinolinato)4-phenylphenolate (BAlq)as the ETL2 (Experimental Devices 29, 31 and 33) or 50 Å of HPT as theETL2 (Experimental Devices 30, 32 and 34) using Ir[5′-Me-5-(4-FPh)ppy]₃(Compound Example VIII) as the emissive material in the emissive layerdoped at 6%, 8% and 10% in CBP.

FIG. 35 shows the plots of the external quantum efficiency (%) vs.current density (mA/cm²) comparing devices with either 100 Å ofaluminum(III)bis(2-methyl-8-hydroxyquinolinato)4-phenylphenolate (BAlq)as the ETL2 (Experimental Devices 29, 31 and 33) or 50 Å of HPT as theETL2 (Experimental Devices 30, 32 and 34) using Ir[5′-Me-5-(4-FPh)ppy]₃(Compound Example VIII) as the emissive material in the emissive layerdoped at 6%, 8% and 10% in CBP.

FIG. 36 shows the normalized electroluminescence spectra (normalized ELintensity vs. wavelength) at a current density of 10 mA/cm² forExperimental Devices 29-34 using Ir[5′-Me-5-(4-FPh)ppy]₃ (CompoundExample VIII) as the emissive material in the emissive layer doped at6%, 8% and 10% in CBP.

FIG. 37 shows the plots of the current density (mA/cm²) vs. the voltage(V) comparing devices with either 100 Å ofaluminum(III)bis(2-methyl-8-hydroxyquinolinato)4-phenylphenolate (BAlq)as the ETL2 (Experimental Devices 35, 37 and 39) or 50 Å of HPT as theETL2 (Experimental Devices 36, 38 and 40) using Ir[5′-Me-5-(3-FPh)ppy]₃(Compound Example IX) as the emissive material in the emissive layerdoped at 6%, 8% and 10% in CBP.

FIG. 38 shows the plots of the luminous efficiency (cd/A) vs. brightness(cd/m²) comparing devices with either 100 Å ofaluminum(III)bis(2-methyl-8-hydroxyquinolinato)4-phenylphenolate (BAlq)as the ETL2 (Experimental Devices 35, 37 and 39) or 50 Å of HPT as theETL2 (Experimental Devices 36, 38 and 40) using Ir[5′-Me-5-(3-FPh)ppy]₃(Compound Example IX) as the emissive material in the emissive layerdoped at 6%, 8% and 10% in CBP.

FIG. 39 shows the plots of the external quantum efficiency (%) vs.current density (mA/cm²) comparing devices with either 100 Å ofaluminum(III)bis(2-methyl-8-hydroxyquinolinato)4-phenylphenolate (BAlq)as the ETL2 (Experimental Devices 35, 37 and 39) or 50 Å of HPT as theETL2 (Experimental Devices 36, 38 and 40) using Ir[5′-Me-5-(3-FPh)ppy]₃(Compound Example VIII) as the emissive material in the emissive layerdoped at 6%, 8% and 10% in CBP.

FIG. 40 shows the normalized electroluminescence spectra (normalized ELintensity vs. wavelength) at a current density of 10 mA/cm² forExperimental Devices 35-40 using Ir[5′-Me-5-(3-FPh)ppy]₃ (CompoundExample VIII) as the emissive material in the emissive layer doped at6%, 8% and 10% in CBP.

FIG. 41 shows the plots of the current density (mA/cm²) vs. the voltage(V) comparing devices with either 100 Å ofaluminum(III)bis(2-methyl-8-hydroxyquinolinato)4-phenylphenolate (BAlq)as the ETL2 (Experimental Devices 61, 63 and 65) or 50 Å of HPT as theETL2 (Experimental Devices 62, 64, and 66) using factris[2-(2′-fluorobiphenyl-3-yl)pyridine]iridium(III) (Compound ExampleX) as the emissive material doped at 6%-10% in CBP.

FIG. 42 shows the plots of luminous efficiency (cd/A) vs. brightness(cd/m²) comparing devices with either 100 Å ofaluminum(III)bis(2-methyl-8-hydroxyquinolinato)4-phenylphenolate (BAlq)as the ETL2 (Experimental Devices 61, 63 and 65) or 50 Å of HPT as theETL2 (Experimental Devices 62, 64 and 66) using factris[2-(2′-fluorobiphenyl-3-yl)pyridine]iridium(III) (Compound ExampleX) as the emissive material doped at 6%-10% in CBP.

FIG. 43 shows the plots of the external quantum efficiency (%) vs.current density (mA/cm²) comparing devices with either 100 Å ofaluminum(III)bis(2-methyl-8-hydroxyquinolinato)4-phenylphenolate (BAlq)as the ETL2 (Experimental Devices 61, 63 and 65) or 50 Å of HPT as theETL2 (Experimental Devices 62, 64 and 66) using factris[2-(2′-fluorobiphenyl-3-yl)pyridine]iridium(III) (Compound ExampleX) as the emissive material in the emissive layer doped at 6%, 8% and10% in CBP.

FIG. 44 shows the normalized electroluminescence spectra (normalized ELintensity vs. wavelength) at a current density of 10 mA/cm² forExperimental devices 61-66 using factris[2-(2′-fluorobiphenyl-3-yl)pyridine]iridium(III) (Compound ExampleX) as the emissive material doped at 6%-10% in CBP.

DETAILED DESCRIPTION

Generally, an OLED comprises at least one organic layer disposed betweenand electrically connected to an anode and a cathode. When a current isapplied, the anode injects holes and the cathode injects electrons intothe organic layer(s). The injected holes and electrons each migratetoward the oppositely charged electrode. When an electron and holelocalize on the same molecule, an “exciton,” which is a localizedelectron-hole pair having an excited energy state, is formed. Light isemitted when the exciton relaxes via a photoemissive mechanism. In somecases, the exciton may be localized on an excimer or an exciplex.Non-radiative mechanisms, such as thermal relaxation, may also occur,but are generally considered undesirable.

The initial OLEDs used emissive molecules that emitted light from theirsinglet states (“fluorescence”) as disclosed, for example, in U.S. Pat.No. 4,769,292, which is incorporated by reference in its entirety.Fluorescent emission generally occurs in a time frame of less than 10nanoseconds.

More recently, OLEDs having emissive materials that emit light fromtriplet states (“phosphorescence”) have been demonstrated. Baldo et al.,“Highly Efficient Phosphorescent Emission from OrganicElectroluminescent Devices,” Nature, vol. 395, 151-154, 1998;(“Baldo-I”) and Baldo et al., “Very high-efficiency green organiclight-emitting devices based on electrophosphorescence,” Appl. Phys.Lett., vol. 75, No. 3, 4-6 (1999) (“Baldo-II”), which are incorporatedby reference in their entireties. Phosphorescence may be referred to asa “forbidden” transition because the transition requires a change inspin states, and quantum mechanics indicates that such a transition isnot favored. As a result, phosphorescence generally occurs in a timeframe exceeding at least 10 nanoseconds, and typically greater than 100nanoseconds. If the natural radiative lifetime of phosphorescence is toolong, triplets may decay by a non-radiative mechanism, such that nolight is emitted. Organic phosphorescence is also often observed inmolecules containing heteroatoms with unshared pairs of electrons atvery low temperatures. 2,2′-bipyridine is such a molecule. Non-radiativedecay mechanisms are typically temperature dependent, such that amaterial that exhibits phosphorescence at liquid nitrogen temperaturesmay not exhibit phosphorescence at room temperature. But, asdemonstrated by Baldo, this problem may be addressed by selectingphosphorescent compounds that do phosphoresce at room temperature.Representative emissive layers include doped or un-doped phosphorescentorgano-metallic materials such as disclosed in U.S. Pat. Nos. 6,303,238and 6,310,360; U.S. Patent Application Publication Nos. 2002-0034656;2002-0182441; and 2003-0072964; and WO-02/074015.

Generally, the excitons in an OLED are believed to be created in a ratioof about 3:1, i.e., approximately 75% triplets and 25% singlets. See,Adachi et al., “Nearly 100% Internal Phosphorescent Efficiency In AnOrganic Light Emitting Device,” J. Appl. Phys., 90, 5048 (2001), whichis incorporated by reference in its entirety. In many cases, singletexcitons may readily transfer their energy to triplet excited states via“intersystem crossing,” whereas triplet excitons may not readilytransfer their energy to singlet excited states. As a result, 100%internal quantum efficiency is theoretically possible withphosphorescent OLEDs. In a fluorescent device, the energy of tripletexcitons is generally lost to radiationless decay processes that heat-upthe device, resulting in much lower internal quantum efficiencies. OLEDsutilizing phosphorescent materials that emit from triplet excited statesare disclosed, for example, in U.S. Pat. No. 6,303,238, which isincorporated by reference in its entirety.

Phosphorescence may be preceded by a transition from a triplet excitedstate to an intermediate non-triplet state from which the emissive decayoccurs. For example, organic molecules coordinated to lanthanideelements often phosphoresce from excited states localized on thelanthanide metal. However, such materials do not phosphoresce directlyfrom a triplet excited state but instead emit from an atomic excitedstate centered on the lanthanide metal ion. The europium diketonatecomplexes illustrate one group of these types of species.

Phosphorescence from triplets can be enhanced over fluorescence byconfining, preferably through bonding, the organic molecule in closeproximity to an atom of high atomic number. This phenomenon, called theheavy atom effect, is created by a mechanism known as spin-orbitcoupling. Such a phosphorescent transition may be observed from anexcited metal-to-ligand charge transfer (MLCT) state of anorganometallic molecule such as tris(2-phenylpyridine)iridium(III).

As used herein, the term “triplet energy” refers to an energycorresponding to the highest energy feature discernable in thephosphorescence spectrum of a given material. The highest energy featureis not necessarily the peak having the greatest intensity in thephosphorescence spectrum, and could, for example, be a local maximum ofa clear shoulder on the high energy side of such a peak.

The term “organometallic” as used herein is as generally understood byone of ordinary skill in the art and as given, for example, in“Inorganic Chemistry” (2nd Edition) by Gary L. Miessler and Donald A.Tarr, Pentice-Hall (1998). Thus, the term organometallic refers tocompounds which have an organic group bonded to a metal through acarbon-metal bond. This class does not include per se coordinationcompounds, which are substances having only donor bonds fromheteroatoms, such as metal complexes of anines, halides, pseudohalides(CN, etc.), and the like. In practice organometallic compounds generallycomprise, in addition to one or more carbon-metal bonds to an organicspecies, one or more donor bonds from a heteroatom. The carbon-metalbond to an organic species refers to a direct bond between a metal and acarbon atom of an organic group, such as phenyl, alkyl, alkenyl, etc.,but does not refer to a metal bond to an “inorganic carbon,” such as thecarbon of CN.

FIG. 1 shows an organic light emitting device 100. The figures are notnecessarily drawn to scale. Device 100 may include a substrate 110, ananode 115, a hole injection layer 120, a hole transport layer 125, anelectron blocking layer 130, an emissive layer 135, a hole blockinglayer 140, an electron transport layer (ETL) 145, an electron injectionlayer 150, a protective layer 155, and a cathode 160. Cathode 160 is acompound cathode having a first conductive layer 162 and a secondconductive layer 164. Device 100 may be fabricated by depositing thelayers described, in order.

Substrate 110 may be any suitable substrate that provides desiredstructural properties. Substrate 110 may be flexible or rigid. Substrate110 may be transparent, translucent or opaque. Plastic and glass areexamples of preferred rigid substrate materials. Plastic and metal foilsare examples of preferred flexible substrate materials. Substrate 110may be a semiconductor material in order to facilitate the fabricationof circuitry. For example, substrate 110 may be a silicon wafer uponwhich circuits are fabricated, capable of controlling OLEDs subsequentlydeposited on the substrate. Other substrates may be used. The materialand thickness of substrate 110 may be chosen to obtain desiredstructural and optical properties.

Anode 115 may be any suitable anode that is sufficiently conductive totransport holes to the organic layers. The material of anode 115preferably has a work function higher than about 4 eV (a “high workfunction material”). Preferred anode materials include conductive metaloxides, such as indium tin oxide (ITO) and indium zinc oxide (IZO),aluminum zinc oxide (AlZnO), and metals. Anode 115 (and substrate 110)may be sufficiently transparent to create a bottom-emitting device. Apreferred transparent substrate and anode combination is commerciallyavailable ITO (anode) deposited on glass or plastic (substrate). Aflexible and transparent substrate-anode combination is disclosed inU.S. Pat. Nos. 5,844,363 and 6,602,540 B2, which are incorporated byreference in their entireties. Anode 115 may be opaque and/orreflective. A reflective anode 115 may be preferred for sometop-emitting devices, to increase the amount of light emitted from thetop of the device. The material and thickness of anode 115 may be chosento obtain desired conductive and optical properties. Where anode 115 istransparent, there may be a range of thickness for a particular materialthat is thick enough to provide the desired conductivity, yet thinenough to provide the desired degree of transparency. Other anodematerials and structures may be used.

Hole transport layer 125 may include a material capable of transportingholes. Hole transport layer 130 may be intrinsic (undoped), or doped.Doping may be used to enhance conductivity. α-NPD and TPD are examplesof intrinsic hole transport layers. An example of a p-doped holetransport layer is m-MTDATA doped with F₄-TCNQ at a molar ratio of 50:1,as disclosed in United States Patent Application Publication No.2002-0071963 A1 to Forrest et al., which is incorporated by reference inits entirety. Other hole transport layers may be used.

Emissive layer 135 may include an organic material capable of emittinglight when a current is passed between anode 115 and cathode 160.Preferably, emissive layer 135 contains a phosphorescent emissivematerial, although fluorescent emissive materials may also be used.Phosphorescent materials are preferred because of the higher luminescentefficiencies associated with such materials. Emissive layer 135 may alsocomprise a host material capable of transporting electrons and/or holes,doped with an emissive material that may trap electrons, holes, and/orexcitons, such that excitons relax from the emissive material via aphotoemissive mechanism. Emissive layer 135 may comprise a singlematerial that combines transport and emissive properties. Whether theemissive material is a dopant or a major constituent, emissive layer 135may comprise other materials, such as dopants that tune the emission ofthe emissive material. Emissive layer 135 may include a plurality ofemissive materials capable of, in combination, emitting a desiredspectrum of light. Examples of phosphorescent emissive materials includeIr(Ppy)₃. Examples of fluorescent emissive materials include DCM andDMQA. Examples of host materials include Alq₃, CBP and mCP. Examples ofemissive and host materials are disclosed in U.S. Pat. No. 6,303,238 toThompson et al., which is incorporated by reference in its entirety.Emissive material may be included in emissive layer 135 in a number ofways. For example, an emissive small molecule may be incorporated into apolymer. For example, an emissive small molecule may be incorporatedinto a polymer. This may be accomplished by several ways: by doping thesmall molecule into the polymer either as a separate and distinctmolecular species; or by incorporating the small molecule into thebackbone of the polymer, so as to form a co-polymer; or by bonding thesmall molecule as a pendant group on the polymer. Other emissive layermaterials and structures may be used. For example, a small moleculeemissive material may be present as the core of a dendrimer.

Many useful emissive materials include one or more ligands bound to ametal center. A ligand may be referred to as “photoactive” if itcontributes directly to the photoactive properties of an organometallicemissive material. A “photoactive” ligand may provide, in conjunctionwith a metal, the energy levels from which and to which an electronmoves when a photon is emitted. Other ligands may be referred to as“ancillary.” Ancillary ligands may modify the photoactive properties ofthe molecule, for example by shifting the energy levels of a photoactiveligand, but ancillary ligands do not directly provide the energy levelsdirectly involved in light emission. A ligand that is photoactive in onemolecule may be ancillary in another. These definitions of photoactiveand ancillary are intended as non-limiting theories.

Electron transport layer (ETL) 140 may include a material capable oftransporting electrons. Electron transport layer 140 may be intrinsic(undoped), or doped. Doping may be used to enhance conductivity. Alq₃ isan example of an intrinsic electron transport layer. An example of ann-doped electron transport layer is BPhen doped with Li at a molar ratioof 1:1, as disclosed in United States Patent Application Publication No.2002-0071963 A1 to Forrest et al., which is incorporated by reference inits entirety. Other electron transport layers may be used.

The charge carrying component of the electron transport layer may beselected such that electrons can be efficiently injected from thecathode into the LUMO (Lowest Unoccupied Molecular Orbital) level of theelectron transport layer. The “charge carrying component” is thematerial responsible for the LUMO that actually transports electrons.This component may be the base material, or it may be a dopant. The LUMOlevel of an organic material may be generally characterized by theelectron affinity of that material and the relative electron injectionefficiency of a cathode may be generally characterized in terms of thework function of the cathode material. This means that the preferredproperties of an electron transport layer and the adjacent cathode maybe specified in terms of the electron affinity of the charge carryingcomponent of the ETL and the work function of the cathode material. Inparticular, so as to achieve high electron injection efficiency, thework function of the cathode material is preferably not greater than theelectron affinity of the charge carrying component of the electrontransport layer by more than about 0.75 eV, more preferably, by not morethan about 0.5 eV. Similar considerations apply to any layer into whichelectrons are being injected.

Cathode 160 may be any suitable material or combination of materialsknown to the art, such that cathode 160 is capable of conductingelectrons and injecting them into the organic layers of device 100.Cathode 160 may be transparent or opaque, and may be reflective. Metalsand metal oxides are examples of suitable cathode materials. Cathode 160may be a single layer, or may have a compound structure. FIG. 1 shows acompound cathode 160 having a thin metal layer 162 and a thickerconductive metal oxide layer 164. In a compound cathode, preferredmaterials for the thicker layer 164 include ITO, IZO, and othermaterials known to the art. U.S. Pat. Nos. 5,703,436, 5,707,745,6,548,956 B2 and 6,576,134 B2, which are incorporated by reference intheir entireties, disclose examples of cathodes including compoundcathodes having a thin layer of metal such as Mg:Ag with an overlyingtransparent, electrically-conductive, sputter-deposited ITO layer. Thepart of cathode 160 that is in contact with the underlying organiclayer, whether it is a single layer cathode 160, the thin metal layer162 of a compound cathode, or some other part, is preferably made of amaterial having a work function lower than about 4 eV (a “low workfunction material”). Other cathode materials and structures may be used.

Blocking layers may be used to reduce the number of charge carriers(electrons or holes) and/or excitons that leave the emissive layer. Anelectron blocking layer 130 may be disposed between emissive layer 135and the hole transport layer 125, to block electrons from leavingemissive layer 135 in the direction of hole transport layer 125.Similarly, a hole blocking layer 140 may be disposed between emissivelayer 135 and electron transport layer 145, to block holes from leavingemissive layer 135 in the direction of electron transport layer 140.Blocking layers may also be used to block excitons from diffusing out ofthe emissive layer. The theory and use of blocking layers is describedin more detail in U.S. Pat. No. 6,097,147 and United States PatentApplication Publication No. 2002-0071963 A1 to Forrest et al., which areincorporated by reference in their entireties.

As used herein, the term “blocking layer” means that the layer providesa barrier that significantly inhibits transport of charge carriersand/or excitons through the device, without suggesting that the layernecessarily completely blocks the charge carriers and/or excitons. Thepresence of such a blocking layer in a device may result insubstantially higher efficiencies as compared to a similar devicelacking a blocking layer. Also, a blocking layer may be used to confineemission to a desired region of an OLED.

Generally, injection layers are comprised of a material that may improvethe injection of charge carriers from one layer, such as an electrode oran organic layer, into an adjacent organic layer. Injection layers mayalso perform a charge transport function. In device 100, hole injectionlayer 120 may be any layer that improves the injection of holes fromanode 115 into hole transport layer 125. CuPc is an example of amaterial that may be used as a hole injection layer from an ITO anode115, and other anodes. In device 100, electron injection layer 150 maybe any layer that improves the injection of electrons into electrontransport layer 145. LiF/Al is an example of a material that may be usedas an electron injection layer into an electron transport layer from anadjacent layer. Other materials or combinations of materials may be usedfor injection layers. Depending upon the configuration of a particulardevice, injection layers may be disposed at locations different thanthose shown in device 100. More examples of injection layers areprovided in U.S. patent application Ser. No. 09/931,948 to Lu et al.,now U.S. Pat. No. 7,071,615, which is incorporated by reference in itsentirety. A hole injection layer may comprise a solution depositedmaterial, such as a spin-coated polymer, e.g., PEDOT:PSS, or it may be avapor deposited small molecule material, e.g., CuPc or MTDATA.

A hole injection layer (HIL) may planarize or wet the anode surface soas to provide efficient hole injection from the anode into the holeinjecting material. A hole injection layer may also have a chargecarrying component having HOMO (Highest Occupied Molecular Orbital)energy levels that favorably match up, as defined by theirherein-described relative ionization potential (IP) energies, with theadjacent anode layer on one side of the HIL and the hole transportinglayer on the opposite side of the HIL. The “charge carrying component”is the material responsible for the HOMO that actually transports holes.This component may be the base material of the HIL, or it may be adopant. Using a doped HIL allows the dopant to be selected for itselectrical properties, and the host to be selected for morphologicalproperties such as wetting, flexibility, toughness, etc. Preferredproperties for the HIL material are such that holes can be efficientlyinjected from the anode into the HIL material. In particular, the chargecarrying component of the HIL preferably has an IP not more than about0.7 eV greater that the IP of the anode material. More preferably, thecharge carrying component has an IP not more than about 0.5 eV greaterthan the anode material. Similar considerations apply to any layer intowhich holes are being injected. HIL materials are further distinguishedfrom conventional hole transporting materials that are typically used inthe hole transporting layer of an OLED in that such HIL materials mayhave a hole conductivity that is substantially less than the holeconductivity of conventional hole transporting materials. The thicknessof the HIL of the present invention may be thick enough to helpplanarize or wet the surface of the anode layer. For example, an HILthickness of as little as 10 nm may be acceptable for a very smoothanode surface. However, since anode surfaces tend to be very rough, athickness for the HIL of up to 50 nm may be desired in some cases.

A protective layer may be used to protect underlying layers duringsubsequent fabrication processes. For example, the processes used tofabricate metal or metal oxide top electrodes may damage organic layers,and a protective layer may be used to reduce or eliminate such damage.In device 100, protective layer 155 may reduce damage to underlyingorganic layers during the fabrication of cathode 160. Preferably, aprotective layer has a high carrier mobility for the type of carrierthat it transports (electrons in device 100), such that it does notsignificantly increase the operating voltage of device 100. CuPc, BCP,and various metal phthalocyanines are examples of materials that may beused in protective layers. Other materials or combinations of materialsmay be used. The thickness of protective layer 155 is preferably thickenough that there is little or no damage to underlying layers due tofabrication processes that occur after organic protective layer 160 isdeposited, yet not so thick as to significantly increase the operatingvoltage of device 100. Protective layer 155 may be doped to increase itsconductivity. For example, a CuPc or BCP protective layer 160 may bedoped with Li. A more detailed description of protective layers may befound in U.S. patent application Ser. No. 09/931,948 to Lu et al., nowU.S. Pat. No. 7,071,615, which is incorporated by reference in itsentirety.

FIG. 2 shows an inverted OLED 200. The device includes a substrate 210,an cathode 215, an emissive layer 220, a hole transport layer 225, andan anode 230. Device 200 may be fabricated by depositing the layersdescribed, in order. Because the most common OLED configuration has acathode disposed over the anode, and device 200 has cathode 215 disposedunder anode 230, device 200 may be referred to as an “inverted” OLED.Materials similar to those described with respect to device 100 may beused in the corresponding layers of device 200. FIG. 2 provides oneexample of how some layers may be omitted from the structure of device100.

The simple layered structure illustrated in FIGS. 1 and 2 is provided byway of non-limiting example, and it is understood that embodiments ofthe invention may be used in connection with a wide variety of otherstructures. The specific materials and structures described areexemplary in nature, and other materials and structures may be used.Functional OLEDs may be achieved by combining the various layersdescribed in different ways, or layers may be omitted entirely, based ondesign, performance, and cost factors. Other layers not specificallydescribed may also be included. Materials other than those specificallydescribed may be used. Although many of the examples provided hereindescribe various layers as comprising a single material, it isunderstood that combinations of materials, such as a mixture of host anddopant, or more generally a mixture, may be used. Also, the layers mayhave various sublayers. The names given to the various layers herein arenot intended to be strictly limiting. For example, in device 200, holetransport layer 225 transports holes and injects holes into emissivelayer 220, and may be described as a hole transport layer or a holeinjection layer. In one embodiment, an OLED may be described as havingan “organic layer” disposed between a cathode and an anode. This organiclayer may comprise a single layer, or may further comprise multiplelayers of different organic materials as described, for example, withrespect to FIGS. 1 and 2.

Structures and materials not specifically described may also be used,such as OLEDs comprised of polymeric materials (PLEDs) such as disclosedin U.S. Pat. No. 5,247,190, Friend et al., which is incorporated byreference in its entirety. By way of further example, OLEDs having asingle organic layer may be used. OLEDs may be stacked, for example asdescribed in U.S. Pat. No. 5,707,745 to Forrest et al, which isincorporated by reference in its entirety. The OLED structure maydeviate from the simple layered structure illustrated in FIGS. 1 and 2.For example, the substrate may include an angled reflective surface toimprove out-coupling, such as a mesa structure as described in U.S. Pat.No. 6,091,195 to Forrest et al., and/or a pit structure as described inU.S. Pat. No. 5,834,893 to Bulovic et al., which are incorporated byreference in their entireties.

Unless otherwise specified, any of the layers of the various embodimentsmay be deposited by any suitable method. For the organic layers,preferred methods include thermal evaporation, ink-jet, such asdescribed in U.S. Pat. Nos. 6,013,982 and 6,087,196, which areincorporated by reference in their entireties, organic vapor phasedeposition (OVPD), such as described in U.S. Pat. No. 6,337,102 toForrest et al., which is incorporated by reference in its entirety, anddeposition by organic vapor jet printing (OVJP), such as described inU.S. patent application Ser. No. 10/233,470, now U.S. Pat. No.7,431,968, which is incorporated by reference in its entirety. Othersuitable deposition methods include spin coating and other solutionbased processes. Solution based processes are preferably carried out innitrogen or an inert atmosphere. For the other layers, preferred methodsinclude thermal evaporation. Preferred patterning methods includedeposition through a mask, cold welding such as described in U.S. Pat.Nos. 6,294,398 and 6,468,819, which are incorporated by reference intheir entireties, and patterning associated with some of the depositionmethods such as ink-jet and OVJD. Other methods may also be used. Thematerials to be deposited may be modified to make them compatible with aparticular deposition method. For example, substituents such as alkyland aryl groups, branched or unbranched, and preferably containing atleast 3 carbons, may be used in small molecules to enhance their abilityto undergo solution processing. Substituents having 20 carbons or moremay be used, and 3-20 carbons is a preferred range. Materials withasymmetric structures may have better solution processibility than thosehaving symmetric structures, because asymmetric materials may have alower tendency to recrystallize. Dendrimer substituents may be used toenhance the ability of small molecules to undergo solution processing.

The molecules disclosed herein may be substituted in a number ofdifferent ways without departing from the scope of the invention. Forexample, substituents may be added to a compound having three bidentateligands, such that after the substituents are added, one or more of thebidentate ligands are linked together via a linking group to form, forexample, a tetradentate or hexadentate ligand having linking group thatcovalently links a first ligand to a second ligand. Other linkages maybe formed. Suitable linking groups and linkages are described, forexample, in U.S. patent application Ser. Nos. 10/771,423 (now abandoned,published as US 2005/0170206) and 10/859,796, now U.S. Pat. No.7,332,232 which are incorporated by reference herein in theirentireties. It is believed that this type of linking may increasestability relative to a similar compound without linking In preferredembodiments, the multidentate ligand systems are prepared by the metalcatalyzed coupling of the linking group to the ligand. See, for example,Beeston et al., Inorg. Chem. 1998, 37, 4368-4379. In a preferredembodiment, the linking group X provides no π-conjugation between thelinked ligands. Having π-conjugation between the linked ligands maychange the electronic properties of the ligands and the resulting metalcomplexes, such as a red-shift in the luminescence. It is desirable tolink the ligands together to without significantly altering theelectronic properties of the ligands and the resulting metal complex. Anon-conjugated linking group may comprise at least one atom in thelinkage which contains no π-electrons, such as an sp³ hybridized carbonor silicon. In a preferred embodiment of the invention, the linkinggroup, X, is selected from the group consisting of —(CR₂)_(d)—,—[O(CR₂)_(e)]O—, or a group having the formula

a. wherein

-   -   A is —(CR₂)_(f)—, or —Z—(CR₂)_(g)—;    -   Z is —O—, —NR—, or —SiR₂—;    -   B¹ is —O—, —NR—, —CR═CR—, aryl, heteroaryl, cycloalkyl, or a        heterocyclic group,

alkyl, aryl, heteroaryl, cycloalkyl, or a heterocyclic group;

a. each R is independently selected from H, alkyl, aralkyl, aryl andheteroaryl,

-   -   i. d is 1 to 6,    -   ii. e is 1 to 6,    -   iii. f is 1 to 4, and    -   iv. g is 1 to 4.

Devices fabricated in accordance with embodiments of the invention maybe incorporated into a wide variety of consumer products, including flatpanel displays, computer monitors, televisions, billboards, lights forinterior or exterior illumination and/or signaling, heads up displays,fully transparent displays, flexible displays, laser printers,telephones, cell phones, personal digital assistants (PDAs), laptopcomputers, digital cameras, camcorders, viewfinders, micro-displays,vehicles, a large area wall, theater or stadium screen, or a sign.Various control mechanisms may be used to control devices fabricated inaccordance with the present invention, including passive matrix andactive matrix. Many of the devices are intended for use in a temperaturerange comfortable to humans, such as 18 degrees C. to 30 degrees C., andmore preferably at room temperature (20-25 degrees C.).

The materials and structures described herein may have applications indevices other than OLEDs. For example, other optoelectronic devices suchas organic solar cells and organic photodetectors may employ thematerials and structures. More generally, organic devices, such asorganic transistors, may employ the materials and structures.

The term “halo” or “halogen” as used herein includes fluorine, chlorine,bromine and iodine.

The term “alkyl” as used herein contemplates both straight and branchedchain alkyl radicals. Preferred alkyl groups are those containing fromone to fifteen carbon atoms and includes methyl, ethyl, propyl,isopropyl, butyl, isobutyl, tert-butyl, and the like. Additionally, thealkyl group may be optionally substituted with one or more substituentsselected from halo, CN, CO₂R, C(O)R, NR₂, cyclic-amino, NO₂, and OR.

The term “cycloalkyl” as used herein contemplates cyclic alkyl radicals.Preferred cycloalkyl groups are those containing 3 to 7 carbon atoms andincludes cyclopropyl, cyclopentyl, cyclohexyl, and the like.Additionally, the cycloalkyl group may be optionally substituted withone or more substituents selected from halo, CN, CO₂R, C(O)R, NR₂,cyclic-amino, NO₂, and OR.

The term “alkenyl” as used herein contemplates both straight andbranched chain alkene radicals. Preferred alkenyl groups are thosecontaining two to fifteen carbon atoms. Additionally, the alkenyl groupmay be optionally substituted with one or more substituents selectedfrom halo, CN, CO₂R, C(O)R, NR₂, cyclic-amino, NO₂, and OR.

The term “alkynyl” as used herein contemplates both straight andbranched chain alkyne radicals. Preferred alkyl groups are thosecontaining two to fifteen carbon atoms. Additionally, the alkynyl groupmay be optionally substituted with one or more substituents selectedfrom halo, CN, CO₂R, C(O)R, NR₂, cyclic-amino, NO₂, and OR.

The terms “alkylaryl” as used herein contemplates an alkyl group thathas as a substituent an aromatic group. Additionally, the alkylarylgroup may be optionally substituted on the aryl with one or moresubstituents selected from halo, CN, CO₂R, C(O)R, NR₂, cyclic-amino,NO₂, and OR. The term “heterocyclic group” as used herein contemplatesnon-aromatic cyclic radicals. Preferred heterocyclic groups are thosecontaining 3 or 7 ring atoms which includes at least one hetero atom,and includes cyclic amines such as morpholino, piperdino, pyrrolidino,and the like, and cyclic ethers, such as tetrahydrofuran,tetrahydropyran, and the like.

The term “aryl” or “aromatic group” as used herein contemplatessingle-ring groups and polycyclic ring systems. The polycyclic rings mayhave two or more rings in which two carbons are common by two adjoiningrings (the rings are “fused”) wherein at least one of the rings isaromatic, e.g., the other rings can be cycloalkyls, cycloalkenyls, aryl,heterocycles and/or heteroaryls.

The term “heteroaryl” as used herein contemplates single-ringhetero-aromatic groups that may include from one to three heteroatoms,for example, pyrrole, furan, thiophene, imidazole, oxazole, thiazole,triazole, pyrazole, pyridine, pyrazine and pyrimidine, and the like. Theterm heteroaryl also includes polycyclic hetero-aromatic systems havingtwo or more rings in which two atoms are common to two adjoining rings(the rings are “fused”) wherein at least one of the rings is aheteroaryl, e.g., the other rings can be cycloalkyls, cycloalkenyls,aryl, heterocycles and/or heteroaryls.

All value ranges, are inclusive over the entire range. Thus, forexample, a range between 0-4 would include the values 0, 1, 2, 3 and 4.

Phosphorescent OLEDs with unexpected and exceptionally high deviceefficiency are disclosed herein. In some embodiments, the phosphorescentdopants used are Ir(5′-alkyl-2-phenylpyridine) type metal complexes.Many alkyl substituted on Ir(2-phenylpyridine) complexes are known.However, we have found that 5′-alkyl substituted analogs have unexpectedproperties such that, when they are incorporated into an organic lightemitting device, unexpected results are attained. In some embodiments,the phosphorescent dopants used are Ir(5′-alkyl substitutedphenyl-isoquinoline) type metal complexes. By incorporating thephosphorescent materials of the present invention into organic lightemitting devices (OLEDs), unexpected and exceptionally high deviceefficiencies have been demonstrated.

In one embodiment of the present invention, a phosphorescent emissivematerial having improved efficiency when incorporated into an organiclight emitting device is provided, the emissive material having theformula I

M is a metal selected from Ir, Pt, Rh or Pd;

-   -   ring A is an aromatic heterocyclic or a fused aromatic        heterocyclic ring having an alkyl substituent at the R′₅        position and having at least one nitrogen atom, N, that is        coordinated to the metal M,    -   wherein the ring A can be optionally substituted with one or        more substituents at the R′₃, R′₄ and R′₆ positions; and        additionally or alternatively the R′₃ and R′₄ substituted        positions on ring A together form, independently a fused ring,        wherein the fused ring may be optionally substituted;    -   ring B is an aromatic ring with at least one carbon atom        coordinated to metal M, wherein ring B can be optionally        substituted with one or more substituents at the R₃, R₄ R₅ and        R₆ positions;    -   R′₃ R′₄ and R′₆ are each independently H, alkyl, alkenyl,        alkynyl, heteroalkyl, alkenyl, alkynyl, heteroalkyl, aryl,        heteroaryl, aralkyl; and wherein R′₃, R′₄ and R′₆ are optionally        substituted by one or more substituents Z; and    -   R₃R₄R₅, R₆ are each independently selected from the group        consisting of H, alkyl, alkenyl, alkynyl, alkylaryl, CN, CO₂R,        C(O)R, NR₂, NO₂, OR, halo, aryl, heteroaryl, substituted aryl,        substituted heteroaryl or a heterocyclic group such that when        R′₃, R′₄, and R′₆ are all H, R₃, R₄, R₅, and R₆ are also all H        or at least one of R₄, R₅ and R₆ is a linking group covalently        linking two or more of the maximum number of ligands that may be        attached to the metal, an unsubstituted phenyl ring, a        fluoro-substituted phenyl ring or a phenyl ring substituted with        a substituent that renders the phenyl ring equally or less        coplanar than the unsubstituted phenyl ring with respect to Ring        B;    -   alternatively, R′₃ and R₆ may be bridged by a group selected        from —CR₂—CR₂—, —CR═CR—, —CR₂—, —O—, —NR—, —O—CR₂—, —NR—CR₂—,        and —N═CR—;    -   each R is independently H, alkyl, alkenyl, alkynyl, heteroalkyl,        aryl, heteroaryl, or aralkyl; wherein R is optionally        substituted by one or more substituents Z;    -   each Z is independently a halogen, R′, OR′, N(R′)₂, SR′, C(O)R′,        C(O)OR′, C(O)N(R′)₂, CN, SO₂, SOR′, SO₂R′, or SO₃R′;    -   Each R′ is independently H, alkyl, alkenyl, alkynyl,        heteroalkyl, aryl, or heteroaryl;    -   (X—Y) is an ancillary ligand;

m is a value from 1 to the maximum number of ligands that may beattached to the metal; and m+n is the maximum number of ligands that maybe attached to the metal.

This embodiment includes a photoactive ligand having the followingligand structure:

This ligand is referred to as “photoactive” because it is believed thatit contributes to the photoactive properties of the emissive material.The emissive material comprises at least one photoactive ligand and aheavy metal ion such that the resulting material has (i) a carbon-metalbond between ring B and the metal and (ii) the nitrogen of ring A iscoordinated to the metal. Thus the emissive materials of Formula Icomprise a partial structure having the following formula

M may be Ir, Pt, Rh or Pd. Preferably, the metal is Ir or Pt. Mostpreferably, the metal is Ir.

Thus in the emissive material of Formula 1:

m, the number of photoactive ligands of a particular type, may be anyinteger from 1 to the maximum number of ligands that may be attached tothe metal. For example, for Ir, m may be 1, 2 or 3. n, the number of“ancillary” ligands of a particular type, may be any integer from zeroto one less than the maximum number of ligands that may be attached tothe metal. (X—Y) represents an ancillary ligand. These ligands arereferred to as “ancillary” because it is believed that they may modifythe photoactive properties of the molecule, as opposed to directlycontributing to the photoactive properties. The definitions ofphotoactive and ancillary are intended as non-limiting theories. Forexample, for Ir, n may be 0, 1 or 2 for bidentate ligands. Ancillaryligands for use in the emissive material may be selected from thoseknown in the art. Non-limiting examples of ancillary ligands may befound in PCT Application Publication WO 02/15645 A1 to Lamansky et al.at pages 89-90, which is incorporated herein by reference. Preferredancillary ligands include acetylacetonate (acac) and picolinate (pic),and derivatives thereof. The preferred ancillary ligands have thefollowing structures:

The emissive materials of Formula I include an emissive material havinga formula where n is zero and m is the maximum number of ligands thatmay be attached to the metal as depicted in the following structure:

For example, for Ir, m is three in this preferred embodiment, and thestructure may be referred to as a “tris” structure. The tris structureis preferred because it is believed to be particularly stable. R₃, R₄,R₅, R₆ R′₃, R′₄, R′₅ and R′₆ are defined according to the definitions ofFormula I.

In one embodiment, m+n is equal to the total number of bidentate ligandsthat may be attached to the metal in question—for example, 3 for Ir. Inanother embodiment, m+n may be less than the maximum number of bidentateligands that may be attached to the metal, in which case otherligands—ancillary, photoactive, or otherwise—may also be attached to themetal.

In another embodiment of the present invention, M is Ir and m is 3,giving an emissive material of the formula:

Where R₃, R₄, R₅, R₆, R′₃, R′₅ and R′₆ are defined according to thedefinitions of Formula I. In some preferred embodiments, particularly inembodiments where green emission is desired, ring A is pyridyl. In otherpreferred embodiments, particularly in embodiments where red emission isdesired, substituents at R′₃ and R′₄ form a fused ring. An example of ared emitting embodiment of the present invention includes an emissivematerial of Formula I having the following structure:

comprising a ligand having the structure:

and a partial structure for the emissive material as follows:

M may be Ir, Pt, Rh or Pd. Preferably, the metal is Ir or Pt. Mostpreferably, the metal is Ir. This embodiment includes an emissivematerial wherein M is iridium and R′₅ is methyl having the structuralformula:

Another preferred embodiment where M is iridium and R′₅ is methylincludes an emissive material having the structural formula:

In one embodiment of the present invention according to Formula I, atleast one of R₃, R₄, R₅, R₆ R′₃, R′₄, and R′₆ is a phenyl substituent.This embodiment includes an emissive material of Formula I where n iszero, and m is the maximum number of ligands that may be attached to themetal as depicted in the following structures:

For example, for Ir, m is three in this preferred embodiment, and thestructure may be referred to as a “tris” structure. The tris structureis preferred because it is believed to be particularly stable. R₃, R₄,R₅, R₆, R′₃, R′₄, R′₅, and R′₆ are defined according to the definitionsof Formula I.

In one embodiment, m+n is equal to the total number of bidentate ligandsthat may be attached to the metal in question—for example, 3 for Ir. Inanother embodiment, m+n may be less than the maximum number of bidentateligands that may be attached to the metal, in which case otherligands—ancillary, photoactive, or otherwise—may also be attached to themetal.

In one embodiment of the emissive materials of Formula I, ring A is anon-fused pyridyl ring, at least one of R₃, R₄, R₅, R₆, R′₃, R′₄, andR′₆ comprises a phenyl moiety. In preferred embodiments at least one ofR₄, R₅, R₆ is an unsubstituted phenyl ring, a fluoro-substituted phenylring or a phenyl ring substituted with a substituent that renders thephenyl ring equally or less coplanar than the unsubstituted phenyl ringwith respect to Ring B. In particular embodiments, the substituent thatrenders the phenyl ring equally or less coplanar than the unsubstitutedphenyl ring with respect to Ring B is an alkyl substituent. Thisembodiment includes an emissive material having the formula

Where R₇ is H, F or a substituent that renders the phenyl substituent atRing B equally coplanar with or less coplanar than the unsubstitutedphenyl ring with respect to Ring B. Preferably R₇ is selected from thegroup consisting of H, F and alkyl.These emissive materials include a ligand structure of the formula

In one embodiment R′₅ is methyl and m is 3, giving an emissive materialof the formula:

In a particularly preferred embodiment, R₅ is unsubstituted phenyl,giving an emissive material of the formula

with a ligand structure having the formula:

and a partial structure for the emissive material of the formula:

-   -   Where R₃, R₄, R₆, R′₃ R′₄, R′₅, and R′₆ are defined according to        the definitions of Formula I.

In another embodiment R₅ is unsubstituted phenyl, M is Ir and m is 3,giving an emissive material of the formula:

Where R₃, R₄, R₆, R′₃ R′₄, R′₅, and R′₆ are defined according to thedefinitions of Formula I. Preferably, R′₅ is methyl, R₅ is unsubstitutedphenyl and R₃═R₄═R₆═R′₃═R′₄═R′₆═H. The emissive material of thisembodiment has the following structure:

In another embodiment, R′₅ is methyl, m is 3, M is Ir and R₅ isalkyl-substituted phenyl, preferably methyl-substituted phenyl giving anemissive material of the formula:

In another embodiment R′₅ is methyl, m is 3, M is Ir and R₅ isfluoro-substituted phenyl giving an emissive material of the formula:

In another embodiment of the emissive materials of Formula I,R₃═R₄═R₅═R₆═R′₃═R′₄═R′₆═H to give an emissive material of the formula

having a ligand structure of the formula:

and a partial structure for the emissive material of the formula:

In a preferred embodiment, n is zero, and m is the maximum number ofligands that may be attached to the metal.

For example, for Ir, m is three in this preferred embodiment, and thestructure may be referred to as a “tris” structure. The tris structureis preferred because it is believed to be particularly stable. R₅ isalkyl as defined in Formula I.

In one embodiment, m+n is equal to the total number of bidentate ligandsthat may be attached to the metal in question—for example, 3 for Ir. Inanother embodiment, m+n may be less than the maximum number of bidentateligands that may be attached to the metal, in which case otherligands—ancillary, photoactive, or otherwise—may also be attached to themetal. Preferably, if there are different photoactive ligands attachedto the metal, each photoactive ligand has the structure indicated inFormula I.

In preferred embodiment, M is Ir and m is 3, giving an emissive materialof the formula:

Wherein R_(5′) is alkyl as defined in Formula I. In a particularlypreferred embodiment, R′₅ is methyl. The emissive material of thisembodiment has the following structure:

and comprises a ligand having the following structure

and a partial structure of for the emissive material as follows:

In another embodiment of Formula I, at least one of R′₃, R′₄ and R′₆ isalkyl in addition to R′₅ being alkyl. In such an embodiment, theremaining positions can be optionally substituted according to thedefinitions of Formula I. This embodiment includes emissive materials inwhich at least one of R′₃, R′₄ and R′₆ is methyl as depicted in thefollowing structural formulas

having corresponding ligand structures as depicted respectively below:

and having partial structures for the emissive material comprisingligand structures of depicted respectively below:

In preferred embodiments, n is zero, and m is the maximum number ofligands that may be attached to the metal as depicted in the structuresbelow:

This embodiment of the invention includes, for example, molecules withthe following structures where M is Ir and m is 3, and R′₅ is methyl:

Another embodiment of the invention comprises an emissive materialhaving a formula in which at least one of R′₃, R′₄, and R′₆ is alkyl andat least one of R₃, R₄, R₅ and R₆ is aryl, preferably phenyl orsubstituted phenyl. This includes an embodiment in which R₅ is a phenylsubstituent and at least one of R′₃, R′₄, and R′₆ is a methylsubstituent. In one embodiment R₅ is phenyl and R′₄ is methyl. Inanother embodiment R₅ is phenyl and R′₃ is methyl. In another embodimentR₅ is phenyl and R′₆ is methyl. These embodiments respectively, give anemissive material comprising a molecule with one of the followingformulas:

each having a corresponding ligand structure as depicted, respectively,below:

and partial structures for the emissive material as depicted,respectively, below:

In other preferred embodiments, n is zero, and m is the maximum numberof ligands that may be attached to the metal. These embodiments includeand emissive material comprising a molecule with the structure:

In one embodiment, the emissive materials comprises a molecule having aformula where M is Ir and m is 3 as depicted below:

Where R_(5′) is defined according to the definitions of Formula I. Inparticular preferred embodiments R′₅ is methyl giving an emissivematerial of the formula:

Particular devices fabricated according to the present invention haveexhibited a maximum external efficiency of 23% (FIG. 14) which webelieve is higher than any thus far reported for OLEDs. Without beinglimited to any theory as to how the present invention works, it isbelieved that the alkyl substituent at position R′₅ as disclosed inFormula I leads to an emissive material resulting in high efficiency andoperational stability when incorporated into an OLED device. In additionto exceptional efficiency, the light emitting devices of the presentinvention may exhibit an operational half-life in excess of about 50hours, or preferably 100 hours, or even more preferably 200 hours atinitial luminance of about 10,700 cd/m² or preferably 12,000 cd/m², oreven more preferably 16,000 cd/m² or most preferably 17,000 cd/m². Wehave previously shown substitution at the R′₃ position to increasedevice lifetime as disclosed in U.S. patent application Ser. No.10/765,295 to Kwong et al., now U.S. Pat. No. 7,279,232, which isincorporated by reference herein in its entirety. Substitution at the R₅position has also been shown to increase device lifetime, as disclosedin U.S. patent application Ser. No. 10/289,915 to Brown et al. (nowabandoned, published as US 2004/0086743), which is also incorporated byreference in its entirety.

In the present invention, alkyl substitution at the R′₅ position showsexceptional efficiency in particular devices. Substitution at the R₅ inaddition to alkyl substitution R′₅ position may show a furtherimprovement in efficiency over alkyl substitution at only the R′₅position. For example, it is believed that specific substituents shownin Formula I provide a particularly efficient molecule, when R₅ isphenyl and R′₅ is methyl, both un-substituted. It is further believedthat the enhanced efficiency is still present if the phenyl and/ormethyl in the R₅ and R′₅ positions, respectively, are substituted.

As used herein, the term “external quantum efficiency” refers to thepercentage of charge carriers injected into a device that result in theemission of a photon from the device in the forward direction. A numberof factors can affect the external quantum efficiency, including the“internal quantum efficiency,” which is the percentage of chargecarriers injected into a device that result in the creation of a photon,and the “outcoupling efficiency,” which is the percentage of photonscreated that are emitted from a device towards a viewer. In someembodiments of the present invention an organic layer comprising a5′alkyl substituted dopant (with and without an aromatic hydrocarbonlayer (HPT) that is in direct contact with an emissive layer) mayenhance the internal quantum efficiency and thus the external quantumefficiency of the device. Because external quantum efficiency is morereadily and directly measured than internal quantum efficiency, it maybe desirable to describe certain aspects of the invention with respectto external quantum efficiency. But, in order to determine whether anenhanced external quantum efficiency is due to the use of an alkylsubstituent at position 5′, it is preferable to account for otherfactors that affect external quantum efficiency. The term “unmodifiedexternal quantum efficiency” as used herein refers to the externalquantum efficiency of a device, after multiplication by a factor toaccount for any differences in the outcoupling efficiency of that deviceand the outcoupling efficiency of the devices described experimentallyherein. For example, a device having an external quantum efficiency of5%, but having an outcoupling efficiency 3 times better than the devicesdescribed herein, would have an “unmodified external quantum efficiency”of 1.33% (one third of 5%). A typical outcoupling efficiency for thetypes of devices described herein is about 20-30%. There are devicestructures having better outcoupling efficiencies than the devicesdescribed herein, and it is anticipated that improvements to outcouplingefficiency will be made over time. Such improvements would enhanceexternal quantum efficiency, but should not affect “unmodified” externalquantum efficiency, and devices having such improvements may fall withinthe scope of the present invention.

“Stability” may be measured in a number of ways. One stabilitymeasurement is the operational stability of the electroluminescentdevice which can be measured in terms of operational half-life. Theoperational half-life is the time required for the luminance of thedevice to decay from the initial luminance (L₀) to 50% of its initialluminance (L_(0.5)) under constant current and at room temperatureunless otherwise noted. Operational half-life depends upon luminance atwhich the device is operated, because a higher luminance generallycorresponds to a faster decay in a particular device. Luminance may bemeasured in cd/m². Devices in accordance with embodiments of the presentinvention can advantageously have an operational half-life in excess ofabout 50 hours, preferably about 100 hours, more preferably about 200hours at initial luminance of about 10,700 cd/m² preferably about 12,000cd/m², more preferably about 16,000 cd/m², most preferably about 17,000cd/m² or higher.

The emissive material of the present invention may comprise a compoundof Formula I such that the device has an unmodified external quantumefficiency of at least about 10% at current densities between about 0.1to about 1000 mA/cm²; and a lifetime of at least about 50 hours at aninitial luminance of at least about 10700 cd/m². In another embodiment,the emissive material may comprise a compound of Formula I such that thedevice has an unmodified external quantum efficiency of at least about15%, preferably at least about 20% at current densities from about 0.1to about 1000 mA/cm²; and a lifetime of at least about 50 hours at aninitial luminance of at least about 10,700 cd/m². In yet anotherembodiment, the emissive layer may be in direct contact with an electrontransport layer comprising a material having a molecular dipole momentless than about 2.0 debyes, such that the device has an external quantumefficiency of at least about 10% at about at current densities betweenabout 0.1 to about 1000 mA/cm².

In one embodiment, it is believed that the use of a second electrontransport layer (ETL2) including an aromatic hydrocarbon having a zeroor low molecular dipole moment (TPD) adjacent to the emissive layer mayfurther enhance device performance, as disclosed in U.S. patentapplication Ser. No. 10/785,287 (now abandoned, published as US2005/0025993) which is incorporated by reference in its entirety herein.Without intending to limit all embodiments with a particular theory ofhow the invention works, it is believed that this symmetric energystructure may improve electron injection from ETL2 into the emissivelayer. The (ETL2) may be in direct contact with the cathode, or theremay be a separate organic layer between the organic enhancement layerand the cathode. Other aromatic hydrocarbon materials may be used.

In some embodiments of the present invention, various alkyl and arylsubstitutions offer a wide range of tunability in terms of evaporationtemperature, solubility, energy levels, device efficiency, and arestable functional groups chemically and in device operation. Forexample, in some embodiments, by appropriately substituting one or morealkyl groups (i.e. methyl, ethyl, t-butyl) on either (or in combination)the pyridine or phenyl rings of a phenyl pyridine ligand and thencyclometallated onto a metal center to give dopant emitters for OLEDdevices, the properties observed gave unexpected improvements inmaterial and device properties including sublimability (lowerdepositions temperatures and remaining residue in the boat), solubility,color tunability, device stability, and device efficiencies. The colorof emission can be fine-tuned to give greater saturation and moredesired CIE color requirements. For example, a 4′,5′-dimethylsubstituted PPY tris Iridium emitter, exhibited a 15 nm blue shift inemission when compared with the unsubstituted Ir(ppy)₃ and a3′,4′-dimethyl substituted PPY tris Iridium emitter exhibited improveddevice stability. Some examples of the alkyl and aryl substitutedemissive materials of the invention have chemical structures as follows:

Compound Structure XIV

XV

XVI

XVII

XVIII

XIX

XX

XXI

XXII

XXIII

XXIV

XXV

XXVI

XXVII

XXVIII

XXIX

XXX

XXXI

It is understood that the various embodiments described herein are byway of example only, and are not intended to limit the scope of theinvention. For example, many of the materials and structures describedherein may be substituted with other materials and structures withoutdeviating from the spirit of the invention. It is understood thatvarious theories as to why the invention works are not intended to belimiting. For example, theories relating to charge transfer are notintended to be limiting.

Material Definitions:

As used herein, abbreviations refer to materials as follows:

CBP: 4,4′-N,N-dicarbazole-biphenyl m-MTDATA4,4′,4″-tris(3-methylphenylphenlyamino)triphenylamine A1q₃:8-tris-hydroxyquinoline aluminum Bphen: 4,7-diphenyl-1,10-phenanthrolinen-BPhen: n-doped BPhen (doped with lithium) F₄-TCNQ:tetrafluoro-tetracyano-quinodimethane p-MTDATA: p-doped m-MTDATA (dopedwith F₄-TCNQ) Ir(ppy)₃: tris(2-phenylpyridine)-iridium Ir(ppz)₃:tris(1-phenylpyrazoloto,N,C(2′)iridium(III) BCP:2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline TAZ:3-phenyl-4-(1′-naphthyl)-5-phenyl-1,2,4-triazole CuPc: copperphthalocyanine ITO: indium tin oxide NPD:N,N′-diphenyl-N-N′-di(1-naphthyl)-benzidine TPD:N,N′-diphenyl-N-N′-di(3-toly)-benzidine BAlq:aluminum(III)bis(2-methy1-8-hydroxyquinolinato)4- phenylphenolate mCP:1,3-N,N-dicarbazole-benzene DCM:4-(dicyanoethylene)-6-(4-dimethylaminostyryl-2-methyl)- 4H-pyran DMQA:N,N′-dimethylquinacridone PEDOT:PSS : an aqueous dispersion ofpoly(3,4-ethylenedioxythiophene) with polystyrenesulfonate (PSS) HPT:2,3,6,7,10,11-hexaphenyltriphenylene

EXPERIMENTAL

Specific representative embodiments of the invention will now bedescribed, including how such embodiments may be made. It is understoodthat the specific methods, materials, conditions, process parameters,apparatus and the like do not necessarily limit the scope of theinvention.

Compound I tris[5-methyl-2-phenylpyridine-N,C^(2′)]iridium (III)[Ir(5′-Meppy)₃] Synthesis Step 1: Synthesis of 3-methyl-6-phenylpyridine

To a 2 L flask, 45.0 g (262 mmol) of 6-bromo-3-methylpyridine, 38.3 g(314 mmol) of phenylboronic acid, 1.47 g (6.54 mmol) of palladiumacetate, 6.86 g (26.2 mmol) of triphenylphosphine and 353 mL of 2M K₂CO₃were added to 405 mL of dimethoxyethane. The mixture was heated atreflux for 20 hours and cooled to room temperature. The aqueous phasewas extracted twice with 200 mL of ethyl acetate. The combined organicextractions were then extracted with brine and dried over magnesiumsulfate. The filtrate was evaporated in vacuo and the resultant oilpurified by Kugelehor distillation (190° C. @ 500 microns) to give 37.2g (84.1% yield) of 3-methyl-6-phenylpyridine as white solids.

Step 2 Synthesis of tris[5-methyl-2-phenylpyridine-NC^(2′)]iridium (III)

To a 100 mL round bottom flask containing 40 mL of ethylene glycoldegassed at 180° C., then cooled to room temperature, was added 3.0 g(17.7 mmol) of 3-methyl-6-phenylpyridine and 2.18 g (4.43 mmol) ofIr(acac)₃. The reaction mixture was stirred for 20 hours at 175° C.under N2. The cooled material was then poured into EtOH and the solidscollected by filtration and rinsed with EtOH. These solids weredissolved in CH₂Cl₂ and purified on a silica gel column usingCH₂Cl₂/hexanes as eluent. The pure fractions were evaporated of solventand the solids recrystallized from CH₂Cl₂/MeOH to give ˜1 g of theproduct after filtration, MeOH rinse and drying. The solids were finallyvacuum evaporated to give 0.50 g of Ir(5′-Meppy)₃ (98.9% assay) and wasconfirmed by NMR as the facial isomer.

Compound II tris[2-(biphenyl-3-yl)-5-methylpyridine]iridium(III)[Ir(5′-Me-5-Phppy)₃] Synthesis Step 1—Synthesis of2-(3-bromophenyl)-5-methylpyridine

2-Bromo-5-methylpyridine (46.1 g, 267 mmol), 3-bromophenylboronic acid(35.8 g, 178 mmol), palladium(II) acetate (1.00 g, 4.4 mmol),triphenylphosphine (4.67 g, 17.8 mmol), and potassium carbonate (67.8 g,491 mmol) were mixed with 370 mL of ethylene glycol dimethyl ether and245 mL of water in a 1000 mL round bottom flask equipped with atemperature probe, reflux condenser, and a magnetic stir bar. Thesolution was heated at reflux under nitrogen for 16 hr. The cooledreaction mixture was then placed in a separatory funnel, and 100 mL ofethyl acetate was added. The aqueous layer was discarded. The organiclayer was extracted twice with a saturated solution of sodium chloride,dried over magnesium sulfate, and evaporated to dryness. After theexcess 2-bromo-5-methylpyridine was distilled off in vacuo at 110° C.,the 2-(3-bromophenyl)-5-methylpyridine was distilled at 200° C. to give30.1 g (68.1% yield) of a slightly orange liquid, which was used for thenext step without further purification.

Step 2 Synthesis of 2-biphenyl-3-yl-5-methylpyridine

2-(3-bromophenyl)-5-methylpyridine (14.0 g, 61 mmol), phenylboronic acid(8.8 g, 72 mmol), palladium (II) acetate (0.34 g, 1.5 mmol),triphenylphosphine (1.6 g, 6.1 mmol), and potassium carbonate (22.3 g,162 mmol) were mixed with 120 mL of ethylene glycol dimethyl ether and80 mL of water in a 500 mL round bottom flask equipped with atemperature probe, reflux condenser, and a magnetic stir bar. Thesolution was then heated at reflux under nitrogen for 16 hr. The cooledreaction mixture was placed in a separatory funnel, and 100 mL of ethylacetate was added. The aqueous layer was discarded. The organic layerwas extracted twice with a saturated solution of sodium chloride, driedover magnesium sulfate, and evaporated to dryness. After the removal ofseveral impurities by vacuum distillation at 115° C., distillation at190° C. gave 13.7 g of 2-biphenyl-3-yl-5-methylpyridine as a viscouslight yellow liquid, which was further purified on a silica gel columnwith ethyl acetate/hexane to give 12.8 g (87.1% yield) of2-biphenyl-3-yl-5-methylpyridine as a white solid.

Step 3—Synthesis oftris[2-(biphenyl-3-yl)-5-methylpyridine-N,C^(2′)]iridium (III)

To a 100 mL three-neck round bottom flask equipped with a temperatureprobe, reflux condenser, nitrogen inlet, and a mechanical stirrer wasadded 30 mL of ethylene glycol. Nitrogen was then bubbled through thesolvent at reflux for 1 hr after which time the2-biphenyl-3-yl-5-methylpyridine (7.75 g, 31.6 mmol) was added. Afterthe solution had become homogeneous, Ir(acac)₃ (3.87 g, 7.9 mmol) wasadded. The reaction mixture was heated at reflux under nitrogen for 24h, resulting in a yellow precipitate. Methanol (60 mL) was added to thecooled reaction mixture, and the precipitate was collected by vacuumfiltration and washed with methanol to give 5.7 g (78.1 yield %) oftris[2-(biphenyl-3-yl)-5-methylpyridine-N,C^(2′)]iridium (III).

Compound III Comparative Example Compound, tris(2-[3-biphenyl]pyridine)Iridium (III): [Ir(5-Phppy)₃] Synthesis

[Ir(5-Phppy)₃] was synthesized by the method described in U.S.Application Publication No. 2004/0086743, Example 1, to givetris(2-[3-biphenyl]pyridine) Iridium (III):

Compound IV factris[1-phenyl-4-methylisoquinolinolato-N,C^(2′)]iridium(III)[Ir(4-Me-1-piq)₃] Synthesis Step 1—Synthesis ofN-(2-phenylpropyl)benzamide

To a solution of 1-amino-2-phenylpropane (25.0 g, 0.185 mol) andtriethylamine (18.2 g, 0.185 mol) in 150 mL of chloroform was addeddropwise, a solution of benzoyl chloride (26.0 g, 0.185 mol) in 150 mLof chloroform under nitrogen. After completion of the addition, thereaction mixture was heated at reflux for 1 hour. The solution was thenwashed with water and the organic layer dried over magnesium sulfate.Removal of the solvent yielded 42.0 g (95%) ofN-(2-phenylpropyl)benzamide as a white powder.

Step 2—Synthesis of 1-phenyl-4-methyl-3,4-dihydroisoquinoline

To a suspension of phosphorus oxychloride (224 g, 136 mL, 1.46 mol) andphosphorus pentoxide (136 g, 0.480 mol) in 410 mL of dry xylene wasadded N-(2-phenylpropyl)benzamide (40 g, 0.167 mol). The suspension washeated with stirring at reflux for 4 h under nitrogen. After cooling toroom temperature, the solvent was decanted off. The reaction vessel wasthen placed in an ice bath, and the residue was dissolved in ice water.Basification with 50% aqueous potassium hydroxide yielded a whiteprecipitate, which was then stirred with dichloromethane and filtered.The solids were discarded. After drying over magnesium sulfate, thedichloromethane was removed by rotary evaporation, yielding 29.0 g (78%)of 1-phenyl-4-methyl-3,4-dihydroisoquinoline as a yellow oil, which wasused for the next reaction without further purification.

Step 3—Synthesis of 1-phenyl-4-methylisoquinoline

To a suspension of activated magnesium dioxide (270 g, 0.132 mol) in 550mL of benzene was added 1-phenyl-4-methyl-3,4-dihydroisoquinoline (29.0g, 0.131 mol) with stirring. The reaction mixture was heated at refluxfor 16 hours. The magnesium dioxide was removed by vacuum filtration andwashed with methylene chloride. Evaporation of the solvent yielded 12.2g (42%) of pure yellow crystals of 1-phenyl-4-methylisoquinoline.

Step 4—Synthesis of Bis[1-phenyl-4-methylisoquinolinato-N,C2′]iridium(III) α-dichloro-bridged dimer

A suspension of 1-phenyl-4-methylisoquinoline (6.0 g, 27.4 mmol) andiridium chloride (5.0 g, 13.7 mmol) in 75 mL of 2-ethoxyethanol and 20mL of water was heated at reflux for 36 h under nitrogen, resulting in ared precipitate, which, after cooling, was collected by vacuumfiltration and washed with methanol, followed by hexane, yielding 6.5 g(67%) of bis[1-phenyl-4-methylisoquinolinato-N,C2′]iridium (III)μ-dichloro-bridged dimer.

Step 5-Bis(1-phenyl-4-methylisoquinolinato-N,C2′)iridium (III)acetylacetonate

A suspension of bis[1-phenyl-4-methylisoquinolinato-N,C2′]iridium (III))μ-dichloro-bridged dimer (6.5 g, 4.9 mmol), acetylacetone (4.9 g, 49mmol), and sodium carbonate (10.3 g, 98 mmol) in 160 mL of2-ethoxyethanol was heated at reflux under nitrogen fro 14 hours. Aftercooling, the product was collected by vacuum filtration and washed withwater, followed by methanol, yielding 2.6 g (37%) ofbis(1-phenyl-4-methylisoquinolinato-N,C2′)iridium acetylacetonate.

Step 6-tris[1-phenyl-4-methylisoquinolinolato-N,C2′]iridium(III)

A suspension ofbis(1-phenyl-4-methylisoquinolinato-N,C2′)iridium(acetylacetonate) (2.3g, 3.1 mmol) and 1-phenyl-4-methylisoquinoline (2.7 g, 12.3 mmol) wereheated with stirring in 50 mL of glycerol under nitrogen for 24 hoursyielding 1.9 g (73%) of crude iridium,tris[1-phenyl-4-methylisoquinolinolato-N,C2′]iridium(III). Purificationon a silica gel column using 70/30 dichloromethane/hexane as the mobilephase yielded 0.9 g (33% yield). The product (375 mg) was then purifiedby vacuum evaporation (Z-1=180° C., Z2=220° C., Z3=280° C., 1×10⁻⁵ torr)yielding 100 mg of the desired product.

Compound V fac tris[3-methyl-5,6dihydrobenzo[h]quinolinato-N,C^(2′)]iridium(III) [Ir(3-Me-dhbq)₃]Synthesis Step 1: 2-methylene-3,4-dihydronaphthalen-1-one

To a suspension of paraformaldehyde (46.2 g, 1.54 mol) andN-methylanilinium trifluoroacetate (TAMA, 46.2 g, 1.54 mol) in 340 mL ofdry THF was added α-tetralone (50 g, 0.342 mol). The solution was heatedat reflux under nitrogen with stirring for 4 h, during which time theparaformaldehyde dissolved. After cooling, diethyl ether (700 mL) wasadded to the reaction mixture. The solvent was separated from thereaction mixture and washed with 500 mL of saturated sodium bicarbonate.Additional diethyl ether was added to the reaction mixture, separatedand used to back extract the aqueous sodium bicarbonate layer. Thecombined organic layers were dried over magnesium sulfate, and thesolution was then concentrated to a volume of approximately 300 mL andfiltered through Celite. Complete evaporation of the ether yielded 50 g(90%) of crude 2-methylene-3,4-dihydronaphthalen-1-one, which was usedimmediately for the next reaction to prevent polymerization of theproduct.

Step 2: 2-ethoxy-3-methyl-3,4,5,6-tetrahydrobenzo[h]chromene

A solution of 2-methylene-3,4-dihydronaphthalen-1-one (44.9 g, 282mmol),tris(6,6,7,7,8,8,8-heptafluoro-2,2-dimethyl-3,5-octanedionato)ytterbium[Yb(fod)₃, 15.0 g, 14.2 mmo], and ethyl propenyl ether (300 g, 390 mL,3.5 mol) in 830 mL of dichloroethane was heated at reflux under nitrogenwith stirring for 20 hours. Evaporation of the solvent left 200 g of abrown liquid that was purified on a silica gel column with 15/85ethylacetate/hexane as the eluent, yielding 140 g of product, which wasused without further purification.

Step 3: 3-methyl-5,6-dihydrobenzo[h]quinoline

To a suspension of hydroxylamine hydrochloride (46.1 g, 0.663 mol) in1070 mL of acetonitrile was added2-ethoxy-3-methyl-3,4,5,6-tetrahydrobenzo[h]chromene (140 g, 0.265 mol)from step two. The reaction mixture was heated at reflux under nitrogenwith stirring for 16 h. Evaporation of the acetonitrile, followed byvacuum distillation of the product yielded 34.5 g of a crude productthat was further purified by silica gel chromatography with 5/95 ethylacetate/hexane as the eluent, yielding 23.2 g (45%) of5,6-dihydrobenzo[h]quinoline as a yellow liquid.

Step 4: factris[3-methyl-5,6-dihydrobenzo[h]quinolinato-N,C^(2′)]iridium(III)

To 50 mL of ethylene glycol at reflux under a nitrogen atmosphere wereadded 10.2 g (52.2 mmol) of 3-methyl-5,6-dihydrobenzo[h]quinoline. Tothis solution were then added 6.4 g (13.1 mmol) of Ir(acac)₃, and thereaction mixture was maintained at reflux for 3 h, resulting in theformation of a yellow precipitate. The mixture was then cooled anddiluted with methanol, and the product was collected by vacuumfiltration and washed with methanol, yielding 6.0 g (59%) of a yellowpowder, which was purified by silica gel column chromatography using70/30 dichloromethane/hexane as the eluent, yielding 3.8 g (37%) ofproduct that was then recrystallized from 140 mL of 1,2-dichlorobenzeneto give 3.3 (32%) of yellow needles. Vacuum evaporation (Z₁=190° C.,Z₂=220° C., Z₃=275° C., 1×10⁻⁵ torr) yielded 2.4 g (24%) of pureproduct.

Compound VI factris[2-(2′-methylbiphenyl-3-yl)pyridinato-N,C2′]iridium(III)(Ir[5′-Me-5-(2-MePh)ppy]₃) Synthesis Step 1:2-(3-bromophenyl)-5-methylpyridine

To a solution of 2-bromo-5-methylpyridine (46.1, 267 mmol),3-bromophenylboronic acid (35.8 g, 178 mmol), palladium (II) acetate(1.0 g, 4.4 mmol), and triphenylphosphine (4.8 g, 18.3 mmol) indimethoxyethane (370 mL) was added a solution of potassium carbonate(67.8 g, 491 mmol) in 245 mL of water. The reaction mixture was heatedat reflux under a nitrogen atmosphere for 16 h and cooled. Ethyl acetatewas added, and the aqueous phase was discarded. Evaporation of theorganic phase after drying over magnesium sulfate yielded a brown liquidfrom which the excess 2-bromo-5-methylpyridine was distilled in vacuo at110° C. Further vacuum distillation at 200° C. yielded 30.1 g (68%) of2-(3-bromophenyl)-5-methylpyridine as a light brown liquid.

Step 2: 2-(2′-methylbiphenyl-3-yl)5-methylpyridine

To a solution of 2-(3-bromophenyl)-5-methylpyridine (26.4 g, 106 mmol),o-tolylboronic acid (17.4 g, 128 mmol), palladium (II) acetate (0.60 g,2.7 mmol), and triphenylphosphine (2.8 g, 10.7 mmol) in 215 mL ofdimethoxyethane was added a solution of potassium carbonate (39.7 g, 287mmol) in 145 mL of water. The reaction mixture was heated at refluxunder a nitrogen atmosphere for 16 h and cooled. Ethyl acetate wasadded, and the aqueous phase was discarded. Evaporation of the organicphase after drying over magnesium sulfate yielded a yellow liquid thatwas then subjected to vacuum distillation at 160° C. to remove most ofthe impurities. Further distillation at 220° C. yielded 29.9 g of acolorless liquid that was further purified by silica gel columnchromatography with 10/90 ethyl acetate/hexane as the eluent to give22.5 g (81%) of pure 2-(2′-methylbiphenyl-3-yl)pyridine as a colorless,viscous liquid.

Step 3 Dimer

To a mixture of 2-ethoxyethanol (95 mL) and water (25 mL) were added11.0 g (42.4 mmol) of 2-(2′-methylbiphenyl-3-yl)pyridine and 7.9 g (21.2mmol) of IrCl₃. The reaction mixture was heated under a nitrogenatmosphere at reflux for 50 h and cooled. The yellow precipitate thatformed was collected by vacuum filtration and washed with methanol andethyl acetate to give 11.0 g (70%) of chloro-bridged dimer

Step 4: factris[2-(2′-methylbiphenyl-3-yl)pyridinato-N,C^(2′)]iridium(III)

A suspension of dimer (11.0 g, 7.4 mmol), silver triflate (14.7 mmol),and 2-(2′-methylbiphenyl-3-yl)pyridine (7.6 g, 29.3 mmol) in 160 mL of2-ethoxyethanol was heated under a nitrogen atmosphere at 95° C. for 60h and then cooled. The precipitate was collected by vacuum filtrationand washed with methanol to give 13.2 g of crude product that was thensubjected to silica gel column chromatography with 50/50dichloromethane/hexane as the eluent, yielding 4.5 g of pure product inaddition to 4.1 g of more impure fractions. The pure fraction wasrecrystallized from 60 mL of a 90/10 toluene/hexane mixture to give 3.4g (24%) of product that was then evaporated in vacuo to yield 0.9 g (6%)of purefac tris[2-(2′-methylbiphenyl-3-yl)pyridine]iridium(III).

Compound VII Hexadentate Ligand Complex Synthesis Step 1:2-(4-bromophenyl)-5-methylpyridine—

To a 500 mL round bottom flask, 4-bromophenylboronic acid (25.0 g,0.125), 2-bromo-5-methylpyridine (20.0 g, 0.114 mol),palladium(0)tetrakistriphenylphosphine (4.0 g, 0.0035 mol), potassiumcarbonate (47.0 g, 0.34 mol.), 1,2-dimethoxyethane (120 mL) and water(120 mL.) were added. The mixture was heated to reflux under nitrogenatmosphere for 18 hours. After the reaction was cooled down, 100 mL ofwater and 150 mL of ethyl acetate were added. The mixture was separatedin a separatory funnel. The organic phases were collected, combined andevaporated. The mixture was distilled using a kugelrohred (spelling?) toobtain 2-(4-bromophenyl)-5-methylpyridine (26.0 g) as a white solid thatwas further purified by recrystallization in hexanes.

Step 2

2-(4-Bromophenyl)-5-methylpyridine (9.3 g, 0.038 mol.) was added to amixture of dry toluene (70 mL), dry diisopropylamine (70 mL),1,3,5-triethynylbenzene (2.0 g, 0.0133 mol),palladium(0)tetrakistriphenylphosphine (1.4 g, 0.0012 mol), CuI (0.15 g)were added in a dry three-necked reaction flask. The mixture was stirredunder nitrogen at room temperature for 3 hours, and then heated up to60° C. for 2 days. The reaction mixture was cooled down and purified bysilica gel column chromatography using dichloromethane/ethyl acetate asthe eluent. The pure fractions were collected and concentrated to giveCompound B (8.0 g) as a white solid.

Step 3

In a computer controlled hydrogenation apparatus, Compound B (10.0 g,0.015 mol.), 5% Pd/C catalyst (5.0 g, 0.0024 mol.) and ˜300 mL of THFwere added in reactor. The reaction was placed under 45 psi of hydrogenpressure and stirred under room temperature overnight. After thereaction was completed the crude product was filtered and the solventconcentrated. The crude product was purified by a silica gel columnchromatography using 30% ethyl acetate in hexane to give Compound C (9.0g) as a white solid.

Step 4: Hexadentate Ligand Complex

Approximately 70 mL of ethylene glycol, Ir(acac)₃, (0.76 mg, 0.00154mol) and Compound C (1.0 g, 0.00151 mol) were added to a 100 mL roundbottom flask. The reaction was heated to 160° C. under nitrogenatmosphere for 24 hours and then cooled down. Methanol was added and theyellow solid collected by vacuum filtration. The crude yellow productwas purified by silica gel column chromatography using 40%dichloromethane in hexane to give the desired compound (900 mg) as ayellow solid.

Compound VIII factris[2-(4′-fluorobiphenyl-3-yl)-5-methylpyridine]iridium(III)(“Ir[5′-Me-5-(4-FPh)ppy]₃”) Synthesis Step 1:2-(3-bromophenyl)-5-methylpyridine

To a 500 mL. reaction flask were added together 2-bromo-5-methylpyridine(46.1 g, 267 mmol), 3-bromophenylboronic acid (35.8 g, 178 mmol),palladium (II) acetate (1.0 g, 4.4 mmol), triphenylphosphine (4.8 g,18.3 mmol), dimethoxyethane (370 mL) and a solution of potassiumcarbonate (67.8 g, 491 mmol) in 245 mL of water. The reaction mixturewas heated at reflux under a nitrogen atmosphere for 16 h and cooled.Ethyl acetate was added, and the aqueous phase was discarded.Evaporation of the organic phase after drying over magnesium sulfateyielded a brown liquid from which the excess 2-bromo-5-methylpyridinewas distilled in vacuo at 110° C. Further vacuum distillation at 200° C.yielded 30.1 g (68%) of 2-(3-bromophenyl)-5-methylpyridine as a lightbrown liquid.

Step 2: 2-(4′-Fluorobiphenyl-3-yl)-5-methylpyridine

In a 500 mL. reaction flask added together2-(3-bromophenyl)-5-methylpyridine (10. g, 40 mmol),4-fluorophenylboronic acid (6.7 g, 48 mmol), palladium (III) acetate(0.22 g, 1.0 mmol), and triphenylphosphine (1 g, 4.0 mmol) in 200 mL ofdimethoxyethane and a solution of potassium carbonate (12.7 g, 120 mmol)in 100 mL of water. The reaction mixture was heated at reflux under anitrogen atmosphere for 16 h and cooled. Ethyl acetate was added, andthe aqueous phase was discarded. The solvent was removed under vacuumand purified by silica gel column chromatography with 50/50 ethylacetate/hexane as the eluents to give 9.0 g (91%) of2-(4′-fluorobiphenyl-3-yl)-5′-methylpyridine as a colorless, viscousliquid.

Step 3: factris[2-(4′-fluorobiphenyl-3-yl)-5-methylpyridine]iridium(III)

To ethylene glycol (100 mL) were added 2.8 g (10.6 mmol) of2-(4′-fluorobiphenyl-3-yl)5-methylpyridine and 1.7 g (3.6 mmol) ofIr(acac)₃. The reaction mixture was heated under a nitrogen atmosphereat reflux for 24 hours and cooled to room temperature. The yellowprecipitate that formed was collected by vacuum filtration and washedwith methanol followed by hexanes to give 1.4 g (40%) of the desiredproduct. The crude product was purified by a silica gel column usingmethylene chloride as the eluent followed by crystallization using2-methoxyethoxyethanol as the solvent.

Compound IX fac tris[2-(3′-fluorobiphenyl-3-yl)pyridine]iridium(III)(“Ir[5′-Me-5-(3-FPh)ppy]₃”) Synthesis Step 1:2-(3′-fluorobiphenyl-3-yl)pyridine

To a 500 mL. three-necked round bottom flask equipped with a stir bar,temperature probe, and a nitrogen inlet were added2-(3-bromophenyl)-5-methylpyridine (14.7 g, 60 mmol),3-fluorophenylboronic acid (10.0 g, 72 mmol), palladium (II) acetate(0.335 g, 1.5 mmol), triphenylphosphine (1.56 g, 6.0 mmol), sodiumcarbonate (17.0 g, 160 mmol), containing dimethoxyethane (120 mL.) andwater (80 mL.). The solution was heated at reflux for twenty hours,cooled, and diluted with ethyl acetate. The organic layer was separated,dried over magnesium sulfate, and evaporated to dryness to give a brownliquid, which was purified by flash silica gel chromatography using a5/95 to 10/90 ethyl acetate/hexane gradient, yielding a viscous,colorless liquid (12.5 g 80%).

Step 2: Synthesis of Dichlorobridge dimer.

To a 250 mL, round-bottomed flask equipped with a stir bar, temperatureprobe, and a nitrogen inlet were combined of2-(3′-fluorobiphenyl-3-yl)pyridine, Iridium chloride (4.4 g, 12 mmol),2-ethoxyethanol (55 mL.), and 15 ml of water (15 mL.). The mixture washeated at reflux for two days. The resulting dimer (8.5 g, 48%) wascollected by vacuum filtration and washed with methanol.

Step 3: fac tris[2-(3′-fluorobiphenyl-3-yl)pyridine]iridium(III)

To a 500 mL, round-bottom flask equipped with a stir bar, temperatureprobe, and a nitrogen inlet was added dimer (8.5 g, 5.7 mmol), silvertriflate (2.9 g, 11.3 mmol), 2-(3′-fluorobiphenyl-3-yl)pyridine (5.9 g,22.5 mmol), and 2-ethoxyethanol (150 mL.). The mixture was heated at 95°C. for 6 days, resulting in a yellow green solid (12 g) that wascollected by vacuum filtration and washed with methanol. Purification ona silica gel column with 70/30 methylene/chloride hexane as the eluentsyielded of fac tris[2-(3′-fluorobiphenyl-3-yl)pyridine]iridium(III) 2.7g (25%) as a yellow powder.

Compound X fac tris[2-(2′-fluorobiphenyl-3-yl)pyridine]iridium(III) Step1: 2-(2′-fluorobiphenyl-3-yl)pyridine

To a 500 mL, three-necked, round-bottomed flask equipped with a stir barand a nitrogen inlet were added 2-(3-bromophenyl)-5-methylpyridine (12.0g, 49 mmol), 2-fluorophenylboronic acid (8.2 g, 58.3 mmol), palladium(I) acetate (0.27 g, 1.2 mmol), triphenylphosphine (1.3 g, 4.8 mmol),sodium carbonate (13.0 g, 131 mmol), water (70 mL.) and dimethoxyethane(100 mL.). The reaction mixture was heated for 20 hours at reflux,cooled, and diluted with ethyl acetate. The organic layer was separated,dried over magnesium sulfate, and evaporated to dryness to give 12.0 gof a dark brown liquid that was purified by flash silica gel columnchromatography using a gradient of 5-10/90-95 ethyl acetate/hexane,yielding 9.6 g of 2-(2′-fluorobiphenyl-3-yl)pyridine as a waxy whitesolid (75%).

Step 2: fac tris[2-(2′-fluorobiphenyl-3-yl)pyridine]iridium(III)

To a 100 mL, round-bottomed flask equipped with a stir bar and anitrogen inlet were added 30 mL of ethylene glycol and2-(2′-fluorobiphenyl-3-yl)pyridine (6.6 g, 25 mmol). The solution washeated to reflux, at which point Ir(acac)₃ (3.1 g 6 mmol) was added. Thereaction mixture was maintained at reflux for two days, after which itwas cooled and diluted with methanol. The resulting yellow solid (5.4 g)was collected by vacuum filtration, washed with ethyl acetate andmethanol, and purified by silica gel column chromatography with 70/30dichloromethane/hexane as the eluent, yielding 3.4 g of material.Recrystallization from 30 mL of benzonitrile gave 1.8 g of pure materialwhich was evaporated in vacuo to yield 1.3 g of factris[2-(2′-fluorobiphenyl-3-yl)pyridine]iridium(III) as yellow crystals.As will be recognized by those of skill in the art, other commerciallyavailable fluoro phenylboronic or difluorophenylboronic acids can beused in step 2, to make additional fluoro substituted regioisomers, forexample:

Compound XI factris[2-(2′,3′-difluorobiphenyl-3-yl)pyridine]iridium(III)

Compound XII factris[2-(2′,4′-difluorobiphenyl-3-yl)pyridine]iridium(III)

Compound XIII factris[2-(2′,5′-difluorobiphenyl-3-yl)pyridine]iridium(III)

Device Fabrication and Measurement

All devices are fabricated by high vacuum (<10⁻⁷ Torr) thermalevaporation. The anode electrode is ˜1200 Å of indium tin oxide (ITO).The cathode consists of 10 Å of LiF followed by 1,000 Å of Al. Alldevices are encapsulated with a glass lid sealed with an epoxy resin ina nitrogen glove box (<1 ppm of H₂O and O₂) immediately afterfabrication, and a moisture getter was incorporated inside the package.Operational lifetests are performed at constant direct current at roomtemperature.

For Experimental Devices 1-10 and Comparative Devices 1-2, the organicstack was fabricated to consist of, sequentially from the ITO surface,CuPc as a hole injection layer (HIL) at a thickness of 100 Å, NPD as ahole transport layer (HTL) at a thickness of 300 Å; CBP doped with 6-12wt % of the dopant emitter (invention compounds and comparativecompounds) as the emissive layer at a thickness of 300 Å. Adjacent tothe emissive layer was an electron transport layer (ETL2) consisting of50 or 100 Å of HPT (Devices 2, 4, 6, 8, 10 and Comparative ExampleDevice 2) or 100 Å of BAlq (Devices 1, 3, 5, 7, 9 and ComparativeExample Device 1). Adjacent to layer ETL2 was another electron transportlayer (ETL1) consisting of Alq₃ at a thickness of 400 or 450 Å.

The luminous efficiency and external quantum efficiency of Devices 1-10and Comparative Example Devices 1 and 2 were measured and are summarizedin Table 1.

TABLE 1 External Luminous quantum efficiency at efficiency DopantCompound % ETL2 ETL1 1000 cd/m² at 1000 cd/m² Device Compound No. doping(thickness) (thickness) (cd/A) (%) 1 Ir(5′-Meppy)₃ I 6 BAlq (100 Å) Alq₃(400 Å) 42.4 12 2 Ir(5′-Meppy)₃ I 6 HPT (100 Å) Alq₃ (400 Å) 40.5 11.3 3Ir(5′-Me-5- II 6 BAlq (100 Å) Alq₃ (400 Å) 50.3 13.6 Phppy)₃ 4Ir(5′-Me-5- II 6 HPT (50 Å) Alq₃ (450 Å) 47.5 12.8 Phppy)₃ 5 Ir(5′-Me-5-II 8 BAlq (100 Å) Alq₃ (400 Å) 44.4 12 Phppy)₃ 6 Ir(5′-Me-5- II 8 HPT(50 Å) Alq₃ (450 Å) 66.3 17.9 Phppy)₃ 7 Ir(5′-Me-5- II 10 BAlq (100 Å)Alq₃ (400 Å) 32.5 8.8 Phppy)₃ 8 Ir(5′-Me-5- II 10 HPT (50 Å) Alq₃ (450Å) 80.1 21.5 Phppy)₃ 9 Ir(5′-Me-5- II 12 BAlq (100 Å) Alq₃ (400 Å) 24.26.5 Phppy)₃ 10  Ir(5′-Me-5- II 12 HPT (50 Å) Alq₃ (450 Å) 81.7 22Phppy)₃ Comparative Ir(5-Phppy)₃ III 7 BAlq (100 Å) Alq₃ (400 Å) 26 6.9Example Device 1 Comparative Ir(5-Phppy)₃ III 6 HPT (50 Å) Alq₃ (450 Å)36.6 9.9 Example Device 2

Very high efficiencies are obtained. It is generally believed that inthin film light emitting devices, due to the optical constraints, onlyabout 20-30% of the light generated inside the device is observedthrough the transparent side(s) of the device. Highest external quantumefficiencies obtained for OLEDs are of phosphorescent types whichreportedly are about 19% [Adachi et al, J. Apply. Phys. 90 (2001) 5048and Ikai et al, Appl. Phys. Lett. 79 (2001) 156]. It can be seen thatexamples 6, 8 and 10 have maximum external quantum efficiencies of20-23%. They represent the highest efficiency OLEDs thus far reported.At 1000 cd/m², the efficiencies of Ir(5-Phppy)₃-doped devices(Comparative Devices 1 and 2) are 6.9% and 9.9% respectively, whereasthose of Ir(5′-Me-5-Phppy)₃-doped Experimental Devices 4 and 5 are 12.8%and 12% respectively. Since they are based on the same device structureand similar emitting dopant concentration, the results indicate that theaddition of the 5′-methyl group plays a role in the efficiencyenhancement. Without being limited to any particular theory of how theinvention works, it is believed that this efficiency improvement may bedue to the improved charge trapping, particularly hole trapping behaviorof the invention compounds. It is further believed that alkyl groupsother than methyl group will have the same effect in efficiencyenhancement. Higher efficiencies are demonstrated byIr(5′-Me-5-Phppy)₃-doped Devices 6 and 8 which utilize HPT as the ETL2compared to Ir(5′-Me-5-Phppy)₃-doped Devices 5 and 7 respectively whichutilize BAlq as the ETL2. Again, without being limited to a particulartheory, it is believed that the enhanced electron injection and/or holeblocking properties of HPT as the ETL2 results in the improvement ofefficiency compared to devices with BAlq as the ETL2.

Table 2 shows the operational stability of the devices using the5′-alkyl substituted invention compounds compared to the devices usingIr(5-Phppy)₃ which has no 5′-alkyl group (Comparative Example Device 1).Device stability was characterized by measuring device luminance as afunction of time under constant current drive of 40 mA/cm² at roomtemperature. While Comparative Example Device 1 exhibited the longestoperational half life [T_((0.5))=300 hours], it operated at lowerinitial luminance (L₀) than the Examples of the present invention, ofabout 9000 cd/m² for Comparative Example Device 1 vs. L₀ of about 12,700cd/m² for experimental Device 1; 10,700 cd/m² for Experimental Device 2;17,000 cd/m² for Experimental Device 5 and 16000 cd/m² for ExperimentalDevice 6.

When scaled to display operation brightness (L₀˜300 cd/m² for redemitting devices and 600 cd/m² for green emitting devices) theoperational half life of Comparative Example Device 1 (a green emittingdevice) is at least 10,000 hours (as disclosed in U.S. ApplicationPublication No. 2004/0086743 which is incorporated by reference hereinin its entirety). Example 5, having a T_((0.5)) of 190 hours with aninitial luminance of about 17000 cd/m², may appear to be shorter-livedthan Comparative Example Device 1. Nonetheless, it operates at a muchhigher brightness (L₀=17000 vs 9000 cd/m²). The T_((0.5))-L₀ product istherefore 300×9000=2.7×10⁶ nit hours and 200×17000=3.23×10⁶ nit.hours,respectively for Comparative Example Device 1 and example 5. It can beseen that example 6 (3.2×10⁶ nit.hours) also has a higher T_((0.5))-L₀product than Comparative Example Device 1. When scaled to displayoperation brightness (L₀˜300 cd/m² and 600 cd/m² for the red and greenemitting devices respectively), the devices in accordance withembodiments of the present invention can advantageously have anoperational half-life in excess of about 10,000 hours. It is believed,therefore, the invention compounds have advantageously very high deviceefficiency and long operational lifetimes. Such properties render themextremely suitable for display and lighting applications.

TABLE 2 Dopant Compound L₀ (cd/m²) at J = 40 T_((0.5)) Device CompoundNo. mA/cm² (hr) T_((0.5)) × L₀ (nit · hour) 1 Ir(5′-Meppy)₃ I 12000 70 8.4 × 10⁵ 2 Ir(5′-Meppy)₃ I 10700 100 1.07 × 10⁶ 5 Ir(5′-Me-5-Phppy)₃II 17000 190 3.23 × 10⁶ 6 Ir(5′-Me-5-Phppy)₃ II 16000 200 3.20 × 10⁶Comparative Ir(5-Phppy)₃ III 9000 300 1.07 × 10⁶ Example Device 1

As demonstrated, devices comprising the present invention compounds havesuperior properties as compared with known devices. While those devicesexemplified in Experimental Devices 1-10 herein are green emittingdevices comprising phenylpyridine type ligands, devices of the presentinvention can emit at any color. For example, phenylisoquinoline ligandscan be coordinated to a metal atom for use in red-emitting devices asdisclosed in U.S. Publication No. 2003/0072964 and U.S. application Ser.No. Ser. No. 10/829/011, now U.S. Pat. No. 7,087,321, incorporatedherein by reference in their entireties. When the substituents taughtherein are incorporated into the emissive material of these devices, itis expected they will similarly exhibit high external quantum andluminous efficiencies and long lifetimes. Accordingly, the presentinvention encompasses methods of increasing device efficiency, such ascan be measured for a device comprising a compound of the presentinvention, relative to that from a device comprising a referencecompound having the same structure, but without the substituents at thesubstitution site(s) disclosed herein.

In further experiments, Experimental Devices 11-26, and 29-40 werefabricated similarly to the Experimental Devices 1-10. ExperimentalDevices 27 and 28 and Comparative Example Devices 3 and 4 were alsosimilarly fabricated except their emissive layers consisted of a neatlayer of emissive material Ir[5′-Me-5-(2-MePh)ppy]₃ for ExperimentalDevices 27 and 28 and Ir(3′-Meppy)₃ for Comparative Example DevicesDevice 3 and 4. The luminous efficiency and external quantum efficiencyof Experimental Devices 11-40 and Comparative Example Devices 3 and 4were measured and are summarized in Table 3.

TABLE 3 External Luminous quantum efficiency efficiency Compound % ETL2ETL1 at 1000 cd/m² at 1000 cd/m² Device Dopant No. doping (thickness)(thickness) (cd/A) (%) 11 Ir(3-Me-dhbq)₃ V 6 BAlq (100 Å) Alq₃ (400 Å)43.4 12 12 Ir(3-Me-dhbq)₃ V 6 HPT (50 Å) Alq₃ (450 Å) 34.8 9.6 13Ir(3-Me-dhbq)₃ V 8 BAlq (100 Å) Alq₃ (400 Å) 48.3 13.3 14 Ir(3-Me-dhbq)₃V 8 HPT (50 Å) Alq₃ (450 Å) 43.2 11.9 15 Ir(3-Me-dhbq)₃ V 10 BAlq (100Å) Alq₃ (400 Å) 40.4 11.1 16 Ir(3-Me-dhbq)₃ V 10 HPT (50 Å) Alq₃ (450 Å)46 12.6 17 Ir[5′-Me-5-(2-MePh)ppy]₃ VI 6 BAlq (100 Å) Alq₃ (400 Å) 47.313 18 Ir[5′-Me-5-(2-MePh)ppy]₃ VI 6 HPT (50 Å) Alq₃ (450 Å) 42.5 11.7 19Ir[5′-Me-5-(2-MePh)ppy]₃ VI 8 BAlq (100 Å) Alq₃ (400 Å) 45.6 12.5 20Ir[5′-Me-5-(2-MePh)ppy]₃ VI 8 HPT (50 Å) Alq₃ (450 Å) 50 13.7 21Ir[5′-Me-5-(2-MePh)ppy]₃ VI 12 BAlq (100 Å) Alq₃ (400 Å) 30.3 8.3 22Ir[5′-Me-5-(2-MePh)ppy]₃ VI 12 HPT (50 Å) Alq₃ (450 Å) 62 17.1 23Hexadentate Ligand Complex VII 6 BAlq (100 Å) Alq₃ (400 Å) 43 11.9 24Hexadentate Ligand Complex VII 6 HPT (50 Å) Alq₃ (450 Å) 38.5 10.7 25Hexadentate Ligand Complex VII 10 BAlq (100 Å) Alq₃ (400 Å) 36.4 10 26Hexadentate Ligand Complex VII 10 HPT (50 Å) Alq₃ (450 Å) 56 15.4 27Ir[5′-Me-5-(2-MePh)ppy]₃ VI 100 BAlq (100 Å) Alq₃ (400 Å) 2.7 0.9 28Ir[5′-Me-5-(2-MePh)ppy]₃ VI 100 HPT (50 Å) Alq₃ (450 Å) 23.3 6.6 Comp.Ir(3′-Meppy)₃ 100 BAlq (100 Å) Alq₃ (400 Å) 1.8 0.6 Example Device 3Comp. Ir(3′-Meppy)₃ 100 HPT (50 Å) Alq₃ (450 Å) 8.8 2.5 Example Device 429 Ir[5′-Me-5-(4-FPh)ppy]₃ VIII 6 BAlq (100 Å) Alq₃ (400 Å) 42 11.6 30Ir[5′-Me-5-(4-FPh)ppy]₃ VIII 6 HPT (50 Å) Alq₃ (450 Å) 42 11.6 31Ir[5′-Me-5-(4-FPh)ppy]₃ VIII 8 BAlq (100 Å) Alq₃ (400 Å) 45 12.4 32Ir[5′-Me-5-(4-FPh)ppy]₃ VIII 8 HPT (50 Å) Alq₃ (450 Å) 53 14.6 33Ir[5′-Me-5-(4-FPh)ppy]₃ VIII 10 BAlq (100 Å) Alq₃ (400 Å) 42 11.6 34Ir[5′-Me-5-(4-FPh)ppy]₃ VIII 10 HPT (50 Å) Alq₃ (450 Å) 60 16.6 35Ir[5′-Me-5-(3-FPh)ppy]₃ IX 6 BAlq (100 Å) Alq₃ (400 Å) 39 10.7 36Ir[5′-Me-5-(3-FPh)ppy]₃ IX 6 HPT (50 Å) Alq₃ (450 Å) 42 11.5 37Ir[5′-Me-5-(3-FPh)ppy]₃ IX 8 BAlq (100 Å) Alq₃ (400 Å) 39 10.7 38Ir[5′-Me-5-(3-FPh)ppy]₃ IX 8 HPT (50 Å) Alq₃ (450 Å) 48 13.2 39Ir[5′-Me-5-(3-FPh)ppy]₃ IX 10 BAlq (100 Å) Alq₃ (400 Å) 37 10.1 40Ir[5′-Me-5-(3-FPh)ppy]₃ IX 10 HPT (50 Å) Alq₃ (450 Å) 47 15.7

Again, the phosphorescent light emitting devices fabricated according tothe present invention showed very high external quantum efficiency.

The voltage is also low (typically <9 V at 10 mA/cm2) for devices havingHPT ETL2. For the Ir(5-Phppy)₃ which has no 5′-alkyl group device thevoltage is about 9.5-10 V for the same device architecture (ComparativeExample Device 2). The driving voltages of the devices with theinvention compounds are about 1 to 1.5 V lower than the devices withpreviously known analogs having no alkyl substitution at the 5′position. These low driving voltages further increase the powerefficiency of the devices

In addition to the high efficiency, the invention compounds alsoevaporate at mild temperatures. Lower evaporation temperatures canreduce damage due to thermal degradation under prolonged heating duringOLED manufacturing by thermal vacuum deposition or other depositionprocesses that require vapor transport of the materials. The evaporatedtemperature of organic materials used in OLEDs is an important aspect inOLED manufacturing. The evaporation temperature is the depositiontemperature at a deposition rate of ˜0.2 Å/s at the substrate under avacuum of <10⁻⁷ torr where the source and the substrate distance isabout 50 cm. Under these conditions, Ir(3-Me-dhbq)₃ (Compound ExampleV), Ir[5′-Me-5-(2-MePh)ppy]₃ (Compound Example VI), and the hexadentateligand complex (Compound Example VII) evaporate at ˜235° C., ˜265° C.and ˜270° C. respectively. These temperatures are lower than that ofIr(5-Phppy)₃ (Comparative Example Compound III) which evaporates at˜300° C. under the same conditions.

Modifications to the 5′alkyl substituted ligands of the presentinvention including further substitutions with certain substituents mayfurther lower the evaporation temperature of the complexes. For example,as shown in the Table 4 (where the phenyl substituent at the 5 positionis designated ring “C”), invention compounds Ir(5′-Me-5-Phppy)₃(Compound Example No. II), Ir[5′-Me-5-(2-MePh)ppy]₃ (Compound ExampleNo. VII), Ir[5′-Me-5-(4-FPh)ppy]₃ (Compound Example No. VIII) andIr[5′-Me-5-(3-FPh)ppy]₃ (Compound Example No. IX) have evaporationtemperatures (T_(evp)) of 315° C., 270° C., 280° C. and 280° C.respectively. As also shown in Table 4, Compound Nos. II, VII and VIIIand IX have dihedral angles between rings B and C of 48°, 87°, 48° and48° respectively.

TABLE 4 Compound Dihedral No. Name Structure angle T_(evp) (° C.) II[Ir(5′-Me-5-Phppy)₃]

48° 315 VII Ir[5′-Me-5-(2-MePh)

87° 270 VIII Ir[5′-Me-5-(4- FPh)ppy]₃

48° 280 IX Ir[5′-Me-5-(3- FPh)ppy]₃

48° 280

It is believed that the lower evaporation temperature exhibited byCompound Example No. VII is due to the twisting between rings B and Cexerted by the steric hindrance from the presence of a bulky methylsubstituent at the 5 position on Ring B. The increased non-coplanarityexerted by such substituent groups reduces intermolecular packing in thesolid state. It is well known that a high degree of intermolecularpacking in organic materials increases the evaporation temperature andreduces the solubility. Therefore, in preferred embodiments, phenyl ringC may have substituents that cause ring C to be equally or lessco-planar with respect to ring B than when ring C is un-substituted.Substituents on ring C that cause rings B and C to be more co-planar,especially substituents resulting in dihedral angles less than 20° maybe less desirable. It is expected that compounds where ring C hasbridging substituents such as

(which have dihedral angles between rings B and C of about 0°) would notlower evaporation temperatures over the compounds where ring C isunsubstituted due to increased molecular packing when the phenyl ringsare coplanar. Thus, preferred substituents are those that result in adihedral angle between Rings B and C of at least 20°, more preferably atleast 45° and most preferably greater than 45°. As also shown in Table4, Examples VIII and IX exhibit evaporation temperatures that are 35° C.lower than that of Example II. While the dihedral angles between Rings Band C are similar for Examples II, VIII and IV (48°), it is believedthat fluorine-containing substituents such as those in Examples VIII andIX can lower the degree of molecular packing in the solid state, hencelowering evaporation temperature in organic materials because fluorogroups have weak van der Waals interactions with regular organic groups.Therefore, fluorine containing substituents are desirable groups tolower the evaporation temperature.

When the compounds of the present invention are designed to phosphoresceas green, conjugated substituents on Ring C may not be desirable.Conjugated substituents, such as a fused benzene or other fused aromaticring, tend to delocalize the electrons in the ligand and lead to a lowertriplet energy for the organometallic complex (resulting in a red-shiftin the phosphorescence). Fused benzene or other aromatic ringsubstituent on ring C is also believed to increase the evaporationtemperature as the fused rings may induce additional molecular packingof the compounds in the solid state. Thus, particularly preferredsubstituents on Ring C are non-conjugated substituents that do not causeelectron delocalization and those that result in increasednon-coplanarity between Rings B and C relative to the degree ofcoplanarity when Ring C (for example, phenyl) is unsubstituted.

In order to study different alkyl and aryl substitution patterns toestablish a structure-property relationship in the materialprocessibility and device characteristics of Ir(ppy)₃ typephosphorescent materials and their PHOLEDs, numerous alkyl and arylsubstituted Ir(ppy)₃ phosphorescent materials were synthesized and OLEDsincorporating them as the dopant emitters were fabricated by vacuumthermal evaporation. Alkyl and aryl substitutions are of particularimportance because they offer a wide range of tunability in terms ofevaporation temperature, solubility, energy levels, device efficiency,and that they are stable functional groups chemically and in deviceoperation.

Compound XIV fac-tris[2-(2′-methylbiphenyl-3-yl)pyridine]Ir(III) Step 1:2-(3-bromophenyl)pyridine

2-bromophenylboronic acid (35.0 g, 0.174 mol) was reacted with2-bromopyridine (55.1 g, 0.348 mol) under standard Suzuki couplingcondition. The excess 2-bromopyridine was removed via vacuumdistillation at 125° C. The product was collected at 180° C. as a lightyellow liquid (29 g, 71%) and used for the next reaction withoutadditional purification.

Step 2: 2-(2′-methylbiphenyl-3-yl)pyridine

2-(3-bromophenyl)pyridine (25.0 g, 0.107 mol) was reacted with2-methylphenylboronic acid (17.4 g, 0.128 mol) under standard Suzukicoupling condition. The product (25 g) was collected by Kugelrohrdistillation at 220° C. as a white solid.

Step 3: fac-tris[2-(2′-methylbiphenyl-3-yl)pyridine]Ir(III)

Ethylene glycol (40 mL) and 2-(2′-methylbiphenyl-3-yl)pyridine (7.7 g,31.4 mmol) were combinded in a 100 mL, three-necked flask equipped witha stir bar, reflux condenser, and a nitrogen inlet and heated to reflux.Ir(acac)₃ (3.8 g, 7.8 mmol) was then added and the reaction mixture wasmaintained at reflux for 24 hours and cooled to room temperature.Methanol (50 mL) was added, and the yellow product (4.1 g) was collectedby vacuum filtration and passed through a silica gel plug with methylenechloride as the eluent to afford 4.0 g of material, which was thenrecrystallized from dichlorobenzene/methanol to give 2.6 g of productwhich was further vacuum sublimed to yield 550 mg of pure material.

Compound XV fac-tris[2-(4′-tert-butylbiphenyl-3-yl)pyridine]Ir(III) Step1: 2-(3-bromophenyl)pyridine

14.2 g (0.088 mol) of 2-borompyridine was added to 19.6 g (0.098 mol) of3-bromophenylboronic acid, 5.2 g (0.0044 mol) of Pd(Ph₃)₄, 31.0 g (0.23mol) of K₂CO₃, 50 mL of DME and 50 mL of water. The reaction mixture wasrefluxed for 20 hours and separated on silica gel column. MS confirmedthe desired product.

Step 2: 2-(4′-tert-butylbiphenyl-3-yl)pyridine

13.7 g (0.058 mol) of 2-(3-bromophenyl)pyridine was added to 12.5 g(0.070 mol) of 4-tert-butylphenylboronic acid, 3.4 g (0.003 mol) ofPd(Ph₃)₄, 22.0 g (0.16 mol) of K₂CO₃, 50 mL of DME and 50 mL of water.The reaction mixture was refluxed for 20 hours and separated on silicagel column. MS confirmed the desired product.

Step 3: fac-tris[2-(4′-tert-butylbiphenyl-3-yl)pyridine]Ir(III)

9.0 g (0.03 mol) of 2-(5-(4-tert-butylphenyl)phenyl)pyridine and 3.8 g(0.0077 mol) of Ir(acac)₃ were mixed with 15 mL of ethylene glycol andheated to reflux under nitrogen for 24 hours. The reaction mixture wascooled down. The product was separated on silica gel column and furtherpurified by zone sublimation. MS and NMR both confirmed the desiredproduct.

Compound XVI fac-tris[2-(3′-methylbiphenyl-3-yl)pyridine]iridium(III)

The synthesis of above title complex was conducted in the same fashionas Compound XV. The difference is in Step 2 that instead of using4-tert-butylphenylboronic acid, 3-methylphenylboronic acid was used forthe Suzuki coupling.

Compound XVII fac-tris[2-(biphenyl-3-yl)-4-methylpyridine]Ir(III) Step1: 2-(biphenyl-3-yl)-4-methylpyridine

9.0 g (0.05 mol) of 2-bromo-4-methylpyridine, 12.0 g (0.06 mol) of3-biphenylboronic acid, 1.7 g (0.0015 mol) of Pd(Ph₃)₄, 19.0 g (0.135mol) of K₂CO₃, 50 mL of DME and 50 mL of water. The reaction mixture wasrefluxed for 20 hours and separated on silica gel column. MS confirmedthe desired product.

Step 2: fac-tris[2-(biphenyl-3-yl)-4-methylpyridine]Ir(III)

3.6 g (0.014 mol) of 2-(biphenyl-3-yl)-4-methylpyridine was mixed with2.1 g (0.004 mol) of Ir(acac)₃ and 12 mL of ethylene glycol. The mixturewas heated to reflux for 24 hours. The product was separated on silicagel column and further purified by zone sublimation method. MS and NMRboth confirmed the desired product.

Compound XVIII fac-tris(2-phenyl-3,5-dimethylpyridine) Ir(III)

The synthesis of above title complex was conducted in the same fashionas Compound XX described below. Instead of using 3,4-lutidine,3,5-lutidine was used as the starting material to make the ligand2-phenyl-3,5-dimethylpyridine andfac-tris(2-phenyl-3,5-dimethylpyridine) Ir(III).

Compound XIX fac-tris[2-(4-methylphenyl)pyridine]iridium(III) Step 1:Dimer

To a mixture of 2-ethoxyethanol (200 mL) and water (50 mL) were added2-(4-methylphenyl)pyridine (15.0 g, 89 mmol) and IrCl₃ (16.4 g, 44mmol). The reaction mixture was heated under nitrogen at reflux for 60hours and cooled. The yellow precipitate that formed was collected byvacuum filtration and washed with methanol to give 20.6 g (82%) ofchloro bridged dimer.

Step 2: fac-tris[2-(4-methylphenyl)pyridine]iridium(III)

A mixture of dimer (8.0 g, 7.1 mmol), silver triflate (3.6 g, 14.2mmol), and 2-(4-methylphenyl)pyridine (4.8 g, 28.4 mmol) in 150 mL of2-ethoxyethanol was heated under nitrogen at 100° C. for 60 hours andthen cooled. The precipitate was collected by vacuum filtration andwashed with methanol to give 9.0 g of crude product, 5.6 of which wasthen dissolved in 7.8 L of 70/30 dichloromethane/hexanes and subjectedto silica gel column chromatography with the same solvent system,yielding 3.4 g of product that was then recrystallized fromdichloroethane, then THF to yield 1.2 g of pure product. Vacuumsublimation of the recrystallized product afforded 900 mg of the pureproduct.

Compound XX and Compound XXI

fac-tris(2-phenyl-4,5-dimethylpyridine) Ir(III) andfac-tris(2-phenyl-3,4-dimethylpyridine) Ir(III)

Step 1: 2-amino-4,5-dimethylpyridine and 2-amino-3,4-dimethylpyridine

To 100.0 g (0.1 mol) of 3,4-lutidine (3,4-dimethylpyridine) and 40 g(1.0 mol) of sodium amide was added 237 mL (1.86 mol) ofN,N-dimethylaniline. The reaction mixture was heated with stirring undernitrogen for 7 hours at 150° C. After cooling down, the reaction mixturewas added to 400 mL of ice and ˜250 mL of ethyl acetate was added to themixture to extract the organic phase. The organic phase was evaporatedand fractional distilled. About ˜40 g (35% yield) of a white solidmixture of 2-amino-3,4-dimethylpyridne (˜78%) and2-amino-4,5-dimethylpyridne (˜22%) was obtained for next step reactionwithout further purification. MS results confirmed the desired products.

Step 2: 2-bromo-4,5-dimethylpyridine and 2-bromo-3,4-dimethylpyridine

8.0 g (0.065 mol) of the Step 1 mixture was added to ˜25 mL of 60% HBr,then stirred at −10° C. to −17° C. 31.0 g (0.2 mol) of pre-cooled Br₂(0° C.) was added dropwise and the mixture was stirred for 20 mins.Pre-cooled (0° C.) NaNO₂ solution of 11.4 g (0.16 mol) of NaNO₂dissolved in 18-20 mL of water was added dropwise into the reactionmixture at about −15° C. After the addition, the reaction was stirredfor one hour. Ice-cooled 25% NaOH solution was added slowly until thesolution became basic. 200 mL of ethyl acetate was added to the mixtureto extract the organic phase. The organic phase solvent was evaporatedand distilled under vacuum. ˜10.7 g (88% yield) a solid mixture of2-bromo-3,4-dimethylpyridne (˜78%) and 2-bromo-4,5-dimethylpyridne(˜22%) was obtained which confirmed by MS. The mixture was used for nextstep Suzuki coupling reaction without further purification.

Step 3: 2-phenyl-4,5-dimethylpyridine and 2-phenyl-3,4-dimethylpyridine

12.0 g (0.0645 mol) of the Step 2 mixture was added to 9.1 g (0.071 mol)of phenylboronic acid, 2.2 g (0.002 mol) of Pd(Ph₃)₄, 24 g (0.174 mol)of K₂CO₃, 100 mL of DME and 100 mL of water. The reaction mixture wasrefluxed for 20 hours and separated on silica gel column with 10% ethylacetate in hexane. ˜7.5 g of 2-phenyl-3,4-dimethylpyridine and 2.5 g of2-phenyl-4,5-dimethylpyridine were obtained which are confirmed by NMRand MS.

Step 4: fac-tris(2-phenyl-4,5-dimethylpyridine) Ir(III) andfac-tris(2-phenyl-3,4-dimethylpyridine) Ir(III)

2.0 g (0.01 mol) of 2-phenyl-4,5-dimethylpyridine was added to 1.6 g(0.003 mol) of Ir(acac)₃ and 10 mL of ethylene glycol and heated toreflux under nitrogen for 20 hours. The reaction mixture was separatedon silica gel column to obtain tris(2-phenyl-4,5-dimethylpyridine)Ir(III) as in Example 46. The complex was further purified by zonesublimation method and confirmed by both MS and NMR.

5.0 g (0.027 mol) of 2-phenyl-3,4-dimethylpyridine was added with 2.2 g(0.0045 mol) of Ir(acac)₃ and heated under nitrogen at high temperaturefor 24 hours. The reaction mixture was separated on silica gel column toobtain tris(2-phenyl-3,4-dimethylpyridine) Ir(III) complex as CompoundXX. The complex was further purified by zone sublimation method andconfirmed by both MS and NMR.

Compound XXII fac-tris[2-(biphenyl-3-yl)-4-tert-butylpyridine]Ir(III)Step 1: 2-amino-4-tert-butylpyridine

To 100 g (0.73 mol) of 4-tert-butyl-pyridine and 31.0 g (0.79 mol) ofsodium amide was added 181 mL (1.5 mol) of N,N,-dimethylaniline. Thereaction mixture was heated with stirring under nitrogen for 7 hours at150° C. After cooling, the reaction mixture was added to 400 mL of ice.˜250 mL of ethyl acetate was added to the mixture to extract the organicphase. The organic phase was evaporated and fractional distilled. About˜50 g (45% yield) of 2-amino-4-tert-butylpyridine was obtained. MSresults confirmed the desired products.

Step 2: 2-bromo-4-tert-butylpyridine

39.0 g (0.26 mol) of 2-amino-4-tert-butyl-pyridine was added with ˜100mL of 60% HBr, then stirred at −10° C. to −17° C. 124.0 g (0.78 mol) ofpre-cooled Br₂ (˜0° C.) was added dropwise added and the mixture wasstirred for 20 mins. Pre-cooled (0° C.) NaNO₂ solution of 46.0 g (0.67mol) of NaNO₂ dissolved in ˜80 mL of water was added dropwise into thereaction mixture at −10° C. to −17° C. After addition, the reaction wasstirred for one hour. Ice-cooled ˜25% NaOH solution was added slowlyuntil the solution became basic. 200 mL of ethyl acetate was added tothe mixture to extract the organic phase. The organic phase solvent wasevaporated and distilled under vacuum. ˜47 g (85% yield) of2-bromo-4-tert-butyl-pyridne was obtained. The product was confirmed byMS.

Step 3: 2-(biphenyl-3-yl)-4-tert-butylpyridine

15.0 g (0.7 mol) of 2-bromo-4-tert-butyl-pyridne was added to 13.8 g(0.7 mol) of 3-biphenylboronic acid, 2.4 g (0.002 mol) of Pd(Ph₃)₄, 26 g(0.188 mol) of K₂CO₃, 100 mL of DME and 100 mL of water. The reactionmixture was refluxed for 20 hours and separated on silica gel columnwith 10% ethyl acetate in hexane. ˜18.0 g (˜90% yield)2-(biphenyl-3-yl)-4-tert-butylpyridine was obtained. MS confirmed thedesired product.

Step 4: fac-tris(2-(3-biphenyl)-4-tert-butylpyridine Ir(III)

7.1 g (0.024 mol) of 2-(biphenyl-3-yl)-4-tert-butylpyridine was added to3.40 g (0.007 mol) of Ir(acac)₃ and ˜10 mL of ethylene glycol. Thereaction mixture was heated to reflux for 24 hours and cooled. ˜5.6 g(˜80% yield) complex was obtained which was separated by silica column.It was further purified by zone sublimation. MS and NMR both confirmedthe desired product.

Compound XXIII fac-tris[2-phenyl-6-methylpyridine]Ir(III) Step 1:2-phenyl-6-methylpyridine

Phenylboronic acid (42.6 g, 0.349 mol) was reacted with2-bromo-6-methylpyridine (50.0 g, 0.290 mol) under standard Suzukicoupling condition. Vacuum distillation of the product (10 mm Hg,135-150° C.) afforded 32.0 g (76%) of2-(4-methylphenyl)-5-methylpyridine as a colorless liquid.

Step 2: fac-tris[2-phenyl-6-methylpyridine]Ir(III)

2-phenyl-6-methylpyridine (32 g, 0.189 mol) was heated to 260° C. in a100 mL, 3-necked flask equipped with a stir bar, nitrogen inlet, and aDean-Stark trap. Ir(acac)₃ (7.7 g, 0.016 mol) was then added, and thereaction mixture was kept at 260° C. for 48 hours while the resultingacetylacetone was distilled off. Methanol (200 mL) was then added, andthe brown precipitate (5.0 g) was collected by vacuum filtration.Purification through a silica gel plug with 70/30 CH₂Cl₂/hexane yielded2.2 g of a yellow solid that was further purified on a basic aluminacolumn with 40-50/60-50 methylene/chloride as the eluent, affording 950mg of pure material. Vacuum sublimation yielded 600 mg of the pureproduct.

Compound XXIV fac-tris[2-(biphenyl-3-yl)-5-tert-butylpyridine]Ir(III)Step 1: 2-(biphenyl-3-yl)-5-bromopyridine

To a 1000 mL, 3-necked flask equipped with a stir bar, nitrogen inlet,and a reflux condenser were added 2,5-dibromopyridine (39.9 g, 84.2mmol), 3-biphenylboronic acid (20.0 g, 101 mmol), palladium(II) acetate(0.47 g, 2.1 mmol), triphenylphosphine (2.2 g, 8.4 mmol), sodiumcarbonate (24.1 g, 227 mmol), dimethoxyethane (170 mL), and water (114mL). The reaction mixture was heated at reflux for 16 h, after whichethyl acetate (200 mL) was added. The organic layer was then dried overmagnesium sulfate and evaporated to dryness. Purification of theresulting solid on a silica gel column with ethyl acetate/hexane as theeluent yielded 19.0 g (73%) of 2-(biphenyl-3-yl)-5-bromopyridine as awhite solid.

Step 2: 2-(biphenyl-3-yl)-5-tert-butylpyridine

To a suspension of copper(I) cyanide (17.3 g, 11 mmol) in 1730 mL ofanhydrous THF in an oven-dried, five liter, three-necked flask equippedwith a mechanical stirrer and a nitrogen inlet was addedtert-butylmagnesium chloride (200 mL, 2.0 M, 400 mmol) via an additionfunnel, and the mixture was stirred at −78° C. for 30 min. A solution of2-(biphenyl-3-yl)-5-bromopyridine (15.0 g, 48 mmol) in 100 mL of dry THFwas then added dropwise, and the mixture was stirred for 3 hours at −78°C. and then overnight at room temperature. Saturated aqueous ammoniumhydroxide was then added to quench the reaction mixture, the pH wasadjusted to 10 with aqueous sodium hydroxide, and the mixture wasextracted with methylene chloride. The organic layer was dried overmagnesium sulfate and evaporated to dryness to give 9.0 g of lightyellow liquid that was purified by flash silica gel chromatography witha 2.5/97.5 to 7.5/92.5 ethyl acetate/hexane gradient as the eluent,affording 2.6 g (19%) of 2-(biphenyl-3-yl)-5-tert-butylpyridine as acolorless liquid.

Step 3: fac-tris[2-(biphenyl-3-yl)-5-tert-butylpyridine]Ir(III)

Ethylene glycol (20 mL) and 2-(biphenyl-3-yl)-5-tert-butylpyridine (2.6g, 9.1 mmol) were combinded in a 100 mL, three-necked flask equippedwith a stir bar, reflux condenser, and a nitrogen inlet and heated toreflux. Ir(acac)₃ (1.1 g, 2.3 mmol) was then added and the reactionmixture was maintained at reflux for 24 hours and cooled to roomtemperature. Methanol (50 mL) was added, and the yellow product (2.1 g)was collected by vacuum filtration and passed through a silica gelcolumn with 70/30 dichloromethane/hexane as the eluent, affording 1.8 gof pure material. Vacuum sublimation yielded 1.0 g of the product.

Compound XXV fac-tris[2-(3′-ethylbiphenyl-3-yl)pyridine]Ir(III) Step 1:3-ethylboronic acid

A solution of 3-ethylbromobenzene (50 g, 270 mmol) in 225 mL ofanhydrous THF was cooled to −78° C. in a two liter, three-necked flaskequipped with a stir bar, nitrogen inlet, and an addition funnel, andn-butyllithium (190 mL, 1.6 M in hexanes) was then added dropwise. Thesolution was allowed to stir for 30 minutes, and trimethylborate (33.7g, 324 mmol) was then added dropwise. The reaction mixture was thenallowed to warm up to room temperature, stirred for 4 hours, andquenched with water. The organic layer was separated and evaporated todryness. The residue was then acidified to pH 1 with concentrated HCl.The solution was diluted with water, and the product was collected byvacuum filtration to afford 24.7 g of 3-ethylboronic acid.

Step 2: 2-(3′-ethylbiphenyl-3-yl)-5-methylpyridine

2-(3-bromophenyl)-5-methylpyridine (16.7 g, 67.3 mmol) was reacted with3-ethylboronic acid (22.7 g, 151 mmol) under standard Suzuki couplingcondition. Kugelrohr distillation at 230° C. yielded 32.0 g of a yellowliquid that was then purified by silica gel column chromatography with10/90 ethyl acetate/hexane as the eluent, affording 17.4 g of2-(3′-ethylbiphenyl-3-yl)pyridine as a clear, viscous liquid.

Step 3: fac-tris[2-(3′-ethylbiphenyl-3-yl)-5-methylpyridine]Ir(III)

Ethylene glycol (20 mL) and 2-(3′-ethylbiphenyl-3-yl)pyridine (5.0 g,18.3 mmol) were combined in a 100 mL, three-necked flask equipped with astir bar, reflux condenser, and a nitrogen inlet and heated to reflux.Ir(acac)₃ (2.2 g, 4.6 mmol) was then added and the reaction mixture wasmaintained at reflux for 24 hours and cooled to room temperature.Methanol (50 mL) was added, and the yellow precipitate (3.1 g) wascollected by vacuum filtration and passed through a silica gel columnwith 40/60 dichloromethane/hexane as the eluent, affording 2.0 g of theproduct

Compound XXVIfac-tris[2-(4′-n-propylbiphenyl-3-yl)-5-methylpyridine]Ir(III) Step 1:2-(4′-n-propylbiphenyl-3-yl)pyridine

2-(3-bromophenyl)-5-methylpyridine (20.0 g, 81.1 mmol) was reacted with4-n-propylphenylboronic acid (15.9 g, 96.7 mmol) under standard Suzukicoupling condition. Kugelrohr distillation at 230° C. yielded 28.5 g ofa viscous yellow liquid that was then purified by silica gel columnchromatography with 15/85 ethyl acetate/hexane as the eluent, affording14.0 g of pure material plus an additional impure fraction.

Step 2: fac-tris[2-(4′-n-propylbiphenyl-3-yl)-5-methylpyridine]Ir(III)

Ethylene glycol (60 mL) and2-(4′-n-propylbiphenyl-3-yl)-5-methylpyridine (13.6 g, 47.3 mmol) werecombined in a 100 mL, three-necked flask equipped with a stir bar,reflux condenser, and a nitrogen inlet and heated to reflux. Ir(acac)₃(5.8 g, 11.8 mmol) was then added and the reaction mixture wasmaintained at reflux for 24 hours and cooled to room temperature.Methanol (50 mL) was added, and the brown precipitate (7.0 g) wascollected by vacuum filtration and passed through a silica gel columnwith 50/50 dichloromethane/hexane as the eluent, affording 5.9 g of theproduct.

Compounds XXVII and XXVIIIfac-tris(2-(biphenyl-3-yl)-3,4-dimethylpyridine) Ir(III) andfac-tris(2-(biphenyl-3-yl)-4,5-dimethylpyridine) Ir(III)

Their syntheses were conducted in the same fashion as for compounds XXIand XX. The difference is that 3-biphenylboronic acid was used in Step 3instead of phenylboronic acid. The productsfac-tris(2-(biphenyl-3-yl)-3,4-dimethylpyridine) Ir(III) was isolated asCompound XXVII and fac-tris(2-(biphenyl-3-yl)-4,5-dimethylpyridine)Ir(III) was isolated as Compound XXVII.

Compound XXIX: fac-tris(2-phenyl-3,4-5-trimethylpyridine) Ir(III) Step1: 2-amino-3,4,5-trimethylpyridine

23.0 g (0.11 mol) of 2-amino-5-bromo-3,4-dimethylpyridine was mixed with14.1 g (0.11 mol) of trimethylboroxin (TMB), 12.9 g (0.011 mol) ofPd(Ph₃)₄, 46.0 g (0.33 mol) of K₂CO₃ and 220 mL of DMF. The reactionmixture was heated under nitrogen at 115° C. for 18 hours. The reactionmixture was purified by silica gel column. The desired product wasconfirmed by MS.

Step 2: 2-bromo-3,4,5-trimethylpyridine

10.3 g (0.075 mol) of 2-amino-3,4,5-trimethylpyridine was added to 30 mLof 60% HBr. The mixture was cooled to −10 to −17° C. and pre-cooled 36.0g (0.23 mol) bromine was added dropwise and the mixture was stirred for20 mins. Pre-cooled (0° C.) NaNO₂ solution of 13.5 g (0.19 mol) NaNO₂dissolved in 20 mL water was added dropwise into the reaction mixture at−10° C.˜−17° C. After addition, the reaction was stirred for one hour.Ice-cooled 25% NaOH solution was added slowly until the solution becamebasic. 200 mL of ethyl acetate was added to the mixture to extract theorganic phase. The organic phase solvent was evaporated and the residuewas distilled under vacuum. ˜13.1 g (87% yield) of the desired productwas obtained which was confirmed by GC/MS.

Step 3: 2-phenyl-3,4,5-trimethylpyridine

10.2 g (0.051 mol) of 2-bromo-3,4,5-trimethylpyridine was mixed with7.86 g (0.061 mol) of phenylboronic acid, 1.8 g (0.0016 mol) ofPd(Ph₃)₄, 19.1 g (0.13 mol) of K₂CO₃ 50 mL of DME and 50 mL of water.The reaction mixture was refluxed for 20 hours and separated on silicagel column. MS confirmed the desired product.

Step 4: fac-tris(2-phenyl-3,4-5-trimethylpyridine) Ir(III)

-   The complex was synthesized in the same fashion as in Step 4 of    Compound XX.

Compound XXX fac-tris[2-(4-methylphenyl)-3-methylpyridine]Ir(III) Step1: 2-(4-methylphenyl)-3-methylpyridine

4-methylphenylboronic acid (20.0 g, 0.147 mol) was reacted with2-bromo-3-methylpyridine (21.1 g, 0.123 mol) under standard Suzukicoupling condition. A small amount of unreacted 2-bromo-3-methylpyridinewas removed via vacuum distillation. Purification via flash silica gelchromatography with a 1/99 to 7.5/92.5 EtOAc/hexane solvent gradientyielded 17.0 g of 2-(4-methylphenyl)-3-methylpyridine as a colorlessliquid.

Step 2: fac-tris[2-(4-methylphenyl)-3-methylpyridine]Ir(III)

2-(4-methylphenyl)-3-methylpyridine (16.6 g, 91 mmol) was heated to 260°C. in a 100 mL, three-necked flask equipped with a stir bar, nitrogeninlet, and a Dean-Stark trap. Ir(acac)₃ (7.4 g, 15 mmol) was then added,and the reaction mixture was kept at 260° C. for 48 hours while theresulting acetylacetone was distilled off. Methanol (200 mL) was thenadded, and the brown precipitate (8.4 g) was collected by vacuumfiltration. Purification on a silica gel column with 70/30 methylenechloride/hexane yielded 3.5 g of a yellow solid that was thenrecrystallized from 50 mL of chlorobenzene to afford 3.1 g (28%) of purematerial which was further purified by vacuum sublimation.

Compound XXXI fac-tris[2-(biphenyl-3-yl)-4-ethylpyridine) Ir(III)

The above title compound was made in the same fashion as Compound XVIIIexcept that 2-bromo-4-ethyl-pyridine was used instead of2-bromo-4-methyl-pyridine in Step 1.

It is important to note that by substituting the phenylpyridine ligandwith larger alkyl groups, such as n-propyl, n-butyl and t-butyl groups,the solubility of the Ir complex is highly increased. For example,compound XXII and compound XXIV are quite soluble in common organicsolvents. ≧8 mg of 49 can be dissolved in 10 mL of toluene. Forcomparison, Ir(ppy)₃, Ir(5-Phppy)₃ and Ir(3′-Meppy)₃ are quite insolubleand only <<4 mg can be dissolved in 10 mL of toluene. It is anticipatedthat these soluble materials are suitable for solution processeddevices. For example, 0.008 g of compound XXII and 0.1 g of mCP may bedissolved in 10 mL toluene to result in a solution which may bespin-coated for device fabrication.

Device Fabrication and Measurement

All devices were fabricated by high vacuum (<10⁻⁷ Torr) thermalevaporation. The anode electrode is ˜1200 Å of indium tin oxide (ITO).The cathode consists of 10 Å of LiF followed by 1,000 Å of Al. Alldevices are encapsulated with a glass lid sealed with an epoxy resin ina nitrogen glove box (<1 ppm of H₂O and O₂) immediately afterfabrication, and a moisture getter was incorporated inside the package.

Device Examples 41-51 and Comparative Examples 5-6

The organic stack consisted of sequentially, from the ITO surface, 100 Åthick of copper phthalocyanine (CuPc) as the hole injection layer (HIL),300 Å of 4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (α-NPD), as thehole transporting layer (HTL), 300 Å of 4,4′-bis(N-carbazolyl)biphenyl(CBP) doped with 6-12 wt % of the dopant emitter (invention compoundsand comparative compounds) as the emissive layer (EML), 50 of HTP as theETL2, and 400 Å of tris(8-hydroxyquinolinato)aluminum (Alq₃) as theETL1. The current-voltage-luminance (IVL) characteristics andoperational lifetimes are measured and summarized in the Table 5. Atypical display brightness level of 1000 cd/m² for green emittingdevices is chosen for the comparison between different devices.

TABLE 5 At 1000 cd/m² Luminous Power At 40 Device T_(sub) λ_(max)efficiency Voltage efficiency mA/cm² cd · hr/m² Device StructureStructure (° C.) (nm) CIE (cd/A) (V) (lm/W) T_(1/2)(hr) (× million)Comparative Example Device 5

G2 6% 310 518 0.31 0.64 38.3 8.2 14.7 500 5.34 Comparative ExampleDevice 6

G2 8% 218 522 0.32 0.62 35.1 7.9 14.0 560 5.45 41

G2 6% 265-275 518 0.30 0.64 29.4 9 10.2 232 2.09 42

G1 6% 340 526 0.33 0.63 25.2 9.3 8.5 276 2.68 43

G2 10%  21-306 522 0.30 0.65 42 7.9 16.8 318 3.91 44

G2 10% 302 516 0.29 0.65 60 6.6 28.5 190 3.04 45

G2 8% 231 526 0.35 0.61 38.5 8.6 17.1 220 2.24 46

G2 9% 225 520 0.30 0.64 24 9.4 7.9 ~400 ~3.04 47

G2 9% 240 506 0.28 0.62 40 6.4 19.6 130 1.79 48

G2 9% 255 520 0.32 0.63 36.4 7.4 15.4 650 6.43 49

G2 12% 330 518 0.31 0.64 53 5.7 29.2 100 1.79 50

G2 9% 226-237 508 0.27 0.63 32.8 7.4 13.9 120 1.2 51

G2 9% 278-284 518 0.31 0.64 45.9 6.6 21.9 100 1.2

It can be seen from Table 5 that some invention compounds show similaror higher efficiency and lifetime versus the comparative examplesIr(5-Phppy)₃ and Ir(3′Meppy)₃. Device Example 48 shows an efficiency of36.4 cd/A at 1000 cd/m² and a device stability of 6.43×10⁷ cd·hr/m²[defined as T_(1/2)(hr)×L₀(cd/m²) where T_(1/2) is the time for theinitial luminance L₀ to drop to 50%] under a constant current drive of40 mA/cm² at room temperature. Under the same condition, Ir(5-Phppy)₃(Comparative Example 5) is 38.3 cd/A and 5.34×10⁷ cd·hr/m², andIr(3′Meppy)₃ (Comparative Example 6) is 35.1 cd/A and 5.45×10⁷ cd·hr/m².

While the present invention is described with respect to particularexamples and preferred embodiments, it is understood that the presentinvention is not limited to these examples and embodiments. For example,the phosphorescent materials may contain stereo and/or structuralisomers. The phosphorescent materials can also be of heterolepticnature, i.e., a metal complex ligating to more than one type of ligand.The present invention as claimed therefore includes variations from theparticular examples and preferred embodiments described herein and setforth below, as will be apparent to one of skill in the art.

1. A compound having the formula:

wherein M is iridium; (X—Y) is an ancillary ligand; m is a value from 1to the maximum number of ligands that may be attached to the metal; andm+n is the maximum number of ligands that may be attached to the metal;ring A is an aromatic heterocyclic or a fused aromatic heterocyclic ringhaving at least one nitrogen atom, N, that is coordinated to the metalM; wherein R′₄ is an alkyl; and wherein R′₃ and R′₆ is eachindependently H, alkyl, alkenyl, alkynyl, heteroalkyl, aryl, heteroaryl,or aralkyl; and wherein R′₅ is H, aryl, or heteroaryl; ring B is anaromatic ring with at least one carbon atom coordinated to metal M,wherein R₇ represents one or more optional substitutions located on anyposition of the phenyl ring attached to ring B, wherein the optional R₇substitutions are all fluoro or all alkyl; R₃, R₄, and R₆ are eachindependently selected from the group consisting of H, alkyl, alkenyl,alkynyl, alkylaryl, CN, CO₂R, C(O)R, NR₂, NO₂, OR, halo, aryl,heteroaryl, substituted aryl, substituted heteroaryl, and a heterocyclicgroup; each R is independently H, alkyl, alkenyl, alkynyl, heteroalkyl,aryl, heteroaryl, or aralkyl; wherein each R is optionally substitutedby one or more substituents Z; each Z is independently a halogen, R′,OR′, N(R′)₂, SR′, C(O)R′, C(O)OR′, C(O)N(R′)₂, CN, SO₂, SOR′, SO₂R′, orSO₃R′; and each R′ is independently H, alkyl, alkenyl, alkynyl,heteroalkyl, aryl, or heteroaryl.
 2. The compound of claim 1, whereinm=3.
 3. The compound of claim 2, wherein the compound has the formula:


4. The compound of claim 1, wherein R′₄ is selected from methyl, ethyl,n-butyl, n-propyl, and t-butyl, and wherein R′₃ is an alkylindependently selected from methyl, ethyl, n-butyl, n-propyl, andt-butyl.
 5. An organic light-emitting device comprising an anode; acathode; and an emissive layer disposed between the anode and thecathode, wherein the emissive layer comprises a compound of claim
 1. 6.The device of claim 5, wherein m=3.
 7. The device of claim 6, whereinthe compound has the formula:


8. The device of claim 5, wherein R′₄ is selected from methyl, ethyl,n-butyl, n-propyl, and t-butyl, and wherein R′₃ is an alkylindependently selected from methyl, ethyl, n-butyl, n-propyl, andt-butyl.
 9. The compound of claim 1, wherein R′₄ is selected frommethyl, ethyl, n-butyl, n-propyl, and t-butyl.
 10. The compound of claim9, wherein R₃, R₄, and R₆ are all H.
 11. The compound of claim 1,wherein R′₅ is H.